[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 b61fb184 which was reverted in 0847c903
because it broke the build.

rdar://118885724



Co-authored-by: Alastair Houghton <ahoughton@apple.com>