tests: Adapt utf8locale to musl

tests: Adapt utf8locale to musl

utf8locale test failed in musl:

    Testing unit: utf8locale
	    NULL input: passed
	    Empty string: passed
	    ASCII text: passed
	    non-ASCII text in UTF-8 locale: passed
	    OTP message in UTF-8 locale: passed
	    non-ASCII text in ISO-8859-2 locale: failed
		    reason: output differs: expected=`��len� �lu�ou�k�' is=`Šíleně žluťoučký'
	    non-ASCII text in C locale: failed
		    reason: correct is NULL, output is not NULL and should be
    Test results: unit = utf8locale, passed = 5, failed = 2, skipped = 0

The cause of "ISO-8859-2 locale" failure was that musl does not yet
fully support locales and handles any character set identifier in
a locale specifier as an alias for UTF-8. This patch fixes the test by
validing a code set the exampled locale actually sets.

"non-ASCII text in C locale" failed because musl performs a lossy
conversion (replaces unrepresantable chacracters with stars) in constrat
to strict conversion done by glibc. POSIX recommends "an
implementation-defined conversion" in that case. Because
_isds_utf82locale is only used for presenting messages, a lossy output
is not a big deal (libisds simply omitts an incovertible part of
a message on glibc). This patch relaxes the test to accept any output.
We keep the test here to cover all code paths.

https://www.openwall.com/lists/musl/2024/05/11/6
https://bugs.gentoo.org/show_bug.cgi?id=928107