Test: `@test_warn` failure messages are unhelpful

[IanButterworth]

julia> using Test

julia> @test_warn r"foo" time()
Test Failed at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-XC9YQX9HH2.0/build/default-honeycrisp-XC9YQX9HH2-0/julialang/julia-release-1-dot-10/usr/share/julia/stdlib/v1.10/Test/src/Test.jl:869
  Expression: contains_warn(read(fname, String), $(Expr(:escape, :(r"foo"))))

ERROR: There was an error during testing

(on 1.10 but it's the same on master)

1. It should show what read(fname, String) returned
2. $(Expr(:escape, :(r"foo"))) is messy
3. The order of the args to contains_warn is the reverse of the macro input, which is confusing
4. The location of the test is within the Test stdlib, not the user test file.
5. The buildpath /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-XC9YQX9HH2.0/build/default-honeycrisp-XC9YQX9HH2-0/julialang/julia-release-1-dot-10/usr/share/julia/stdlib/v1.10/Test/src/Test.jl:869 should be fixed-up