[libc++] Fix tests on musl (#85085) (#86934)
One or two of the tests need slight tweaks to make them pass when
building with musl.
This patch is a re-application of b61fb18 which was reverted in 0847c90
because it broke the build.
rdar://118885724
Co-authored-by: Alastair Houghton <ahoughton@apple.com>
