Don't hide format string from the compiler

[AlexGuteniev]

Apparently previously compiler's static analysis did not understand %a, so the string was hidden.

Now let's show it!

It fixes warning C4774: 'printf' : format string expected in argument 1 is not a string literal.
Currently the warning is disabled, as it is a warning disabled by default.

Salvages the useful part from #2142

[StephanTLavavej]

Thanks! Although we generally avoid spending time cleaning up the legacy tr1 test suite (as that would be a time sink with little benefit - the tests aren't actively evolving, and aren't used as precedent for writing new tests), this is a clear improvement that's obviously correct and small, so it's worth making an exception.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for simplifying this test code! ✨ 🎉 😸