at_test_nowarn does not suppress the content of stderr anymore

[ranocha]

Currently, I get something like

julia> using Test

julia> @test_nowarn println(stderr, "Some informative warning helpful for debugging failing tests")
Test Failed at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/Test/src/Test.jl:636
  Expression: contains_warn(read(fname, String), $(Expr(:escape, :(r"^(?!.)"s))))
ERROR: There was an error during testing

This doesn't really help much when debugging failing tests. This PR changes the output to

julia> using Test

julia> @test_nowarn println(stderr, "Some informative warning helpful for debugging failing tests")
Some informative warning helpful for debugging failing tests
Test Failed at /home/hendrik/Software/julia/usr/share/julia/stdlib/v1.7/Test/src/Test.jl:702
  Expression: isempty(stderr_content)
   Evaluated: isempty("Some informative warning helpful for debugging failing tests\n")
ERROR: There was an error during testing

This is helpful for me when trying to debug failing tests in packages using @test_nowarn.

[ranocha]

I don't see how the changes of this PR could cause the test failure.

[ranocha]

An appropriate label might be error massages(but I can't add labels).

[vtjnash]

Sorry for the review delay. This sound good to me. I made some minor formatting changes, and then will merge when CI finishes.