[libcxx] [test] Account for differences in a trailing slash in weakly_canonical

This seems to be a documented quirk in libc++'s implementation of
weakly_canonical (in a comment in the weakly_canonical test).
Together with a difference between windows and posix regarding whether
paths can go through nonexistent dirs, this results in a difference in
a trailing slash.

Just document this as expected, and degrade the comment from fixme to
a note, as MS STL and libstdc++ behave in the same way.

Differential Revision: https://reviews.llvm.org/D98642