bpo-32002: Refactor C locale coercion tests (GH-4369)

Exactly which locale requests will end up giving
you the "C" locale is actually platform dependent.

A blank locale and "POSIX" will translate to "C"
on most Linux distros, but may not do so on other platforms, so this adjusts the way the tests are structured to better account for that.

This is an initial step towards fixing the current
test failure on Cygwin (hence the issue reference)