observed failure of TestPkg thread test

[vtjnash]

This is a test that calling using TestPkg on multiple threads will not result in accidentally emitting "Replacing module" text. However it seems broken at the moment (on f4b81af), as it can be made to fail by corrupting the test in progress:

131 vtjnash@deepsea4:~/julia/test$ JULIA_NUM_THREADS=8 ~/rr/bin/rr ../julia threads_exec.jl                                                                                                                                  
rr: Saving execution to trace directory `/home/vtjnash/.local/share/rr/julia-74'.                                                                                                                                            
Precompiling TestPkg...                                                                                       
  1 dependency successfully precompiled in 2 seconds                                                          
┌ Warning: The call to compilecache failed to create a usable precompiled cache file for TestPkg [69145d58-7df6-11e8-0660-cf7622583916]                                                                                      
│   exception = Error reading package image file.                                                                                                                                                                            
└ @ Base loading.jl:2454                                                                                                                                                                                                     
┌ Warning: Replacing module `TestPkg`                                                                                                                                                                                        
└ @ Base loading.jl:2311                                                                                                                                                                                                     
┌ Warning: The call to compilecache failed to create a usable precompiled cache file for TestPkg [69145d58-7df6-11e8-0660-cf7622583916]                                                                                      
│   exception = Error reading package image file.                                                                                                                                                                            
└ @ Base loading.jl:2454                                                                                                                                                                                                     
┌ Warning: Replacing module `TestPkg`                                                                                                                                                                                        
└ @ Base loading.jl:2311                                                                                      
┌ Warning: The call to compilecache failed to create a usable precompiled cache file for TestPkg [69145d58-7df6-11e8-0660-cf7622583916]                                                                                      
│   exception = Error reading package image file.                                                             
└ @ Base loading.jl:2454                                                                                      
┌ Warning: Replacing module `TestPkg`                                                                         
└ @ Base loading.jl:2311                                                                                      
┌ Warning: The call to compilecache failed to create a usable precompiled cache file for TestPkg [69145d58-7df6-11e8-0660-cf7622583916]                                                                                      
│   exception = Error reading package image file.                                                             
└ @ Base loading.jl:2454                                                                                      
┌ Warning: Replacing module `TestPkg`                                                                         
└ @ Base loading.jl:2311                                                                                      
┌ Warning: The call to compilecache failed to create a usable precompiled cache file for TestPkg [69145d58-7df6-11e8-0660-cf7622583916]                                                                                      
│   exception = Error reading package image file.                                                             
└ @ Base loading.jl:2454                                                                                                                                                                                                     
┌ Warning: Replacing module `TestPkg`                                                                                                                                                                                        
└ @ Base loading.jl:2311                                                                                      
┌ Warning: The call to compilecache failed to create a usable precompiled cache file for TestPkg [69145d58-7df6-11e8-0660-cf7622583916]                                                                                      
│   exception = Error reading package image file.                                                             
└ @ Base loading.jl:2454                                                                                      
┌ Warning: Replacing module `TestPkg`                                                                         
└ @ Base loading.jl:2311                                                                                                                                                                                                     
┌ Warning: The call to compilecache failed to create a usable precompiled cache file for TestPkg [69145d58-7df6-11e8-0660-cf7622583916]                                                                                      
│   exception = Error reading package image file.
└ @ Base loading.jl:2454                                                                                      
┌ Warning: Replacing module `TestPkg`                                                                                                                                                                                        
└ @ Base loading.jl:2311