Ensure default state in test is restored

Otherwise tests sometimes fail like so:
```
TestGemInstall#test_user_bundler:
LoadError: 126: The specified module could not be found.   - c:/ri2-ruby/lib/ruby/gems/4.1.0+0/gems/ffi-1.17.3/lib/ffi_c.so
    c:/ri2-ruby/lib/ruby/gems/4.1.0+0/gems/ffi-1.17.3/lib/ffi.rb:5:in 'Kernel#require'
    c:/ri2-ruby/lib/ruby/gems/4.1.0+0/gems/ffi-1.17.3/lib/ffi.rb:5:in '<top (required)>'
    c:/ri2-ruby/lib/ruby/gems/4.1.0+0/gems/win32-process-0.10.0/lib/win32/process/functions.rb:6:in 'Kernel#require'
    c:/ri2-ruby/lib/ruby/gems/4.1.0+0/gems/win32-process-0.10.0/lib/win32/process/functions.rb:6:in '<top (required)>'
    c:/ri2-ruby/lib/ruby/gems/4.1.0+0/gems/win32-process-0.10.0/lib/win32/process.rb:1:in 'Kernel#require_relative'
    c:/ri2-ruby/lib/ruby/gems/4.1.0+0/gems/win32-process-0.10.0/lib/win32/process.rb:1:in '<top (required)>'
    test/test_gem_install.rb:69:in 'Kernel#require'
    test/test_gem_install.rb:69:in 'TestGemInstall#with_test_user'
    test/test_gem_install.rb:122:in 'TestGemInstall#test_user_bundler'
```

Author: larskanis

test/ruby_installer/test_module.rb

@@ -157,6 +157,8 @@ def test_enable_dll_search_paths_with_msys_installed
 
     vars2 = %w[PATH RI_DEVKIT MSYSTEM].map{|var| ENV[var] }
     assert_equal vars1, vars2, "env variables should be unchanged"
+  ensure
+    RubyInstaller::Runtime.enable_dll_search_paths
   end
 
   def test_enable_dll_search_paths_without_msys_installed