gh-90548: Allow Alpine/MUSL to pass test_c_locale_coercion. (GH-134454)

Like cygwin, MUSL defaults to utf-8 if no variables are set.  I have no
idea if the existing tests pass on cygwin, but I made the modifications
such that I shouldn't break it if is.  The additional checks needed for
MUSL are guarded by DEFAULT_LOCALE_IS_C being False.  Based on this
flag, we expect utf-8 for the encodings and no coercion message, as
long as LC_ALL is not set to C.  (That looks like a bit of an issue with
the test structure, but I'm not going to attempt to "fix" that.)
DEFAULT_ENCODING is intentionally not given a default since it is only
used when DEFAULT_LOCALE_IS_C is False, and if you use the flag you'll
need to set it.

After reading through issue 30672, looking at the source, and running a
test on Android, I *think* the current situation is that coercion will
be done if the local is set to POSIX regardless of platform.  However,
if the platform doesn't make POSIX equivalent to C, the encodings when
coercion is disabled will not be the same as for C (it is utf-8 on
android, for example).  This means the tests would fail if POSIX were
added unconditionally to the EXPECTED_C_LOCALE_EQUIVALENTS as envisioned
in the issue.  This *could* be fixed with another flag, but I'm not sure
it is worth the effort.  I'm not even sure Python is behaving optimally
in this case (assuming my analysis is correct).  So I just altered the
comment and add POSIX if and only if the platform is linux.