gh-109989: Fix test_c_locale_coercion when PYTHONIOENCODING is set (#113378)

* gh-109989: Fix test_c_locale_coercion when PYTHONIOENCODING is set

This fixes the existing tests when PYTHONIOENCODING is
set by unsetting PYTHONIOENCODING.

Also add a test that explicitly checks what happens
when PYTHONIOENCODING is set.

Co-authored-by: Nikita Sobolev <mail@sobolevn.me>