[libcxx] [test] Allow C:\System Volume Information to be missing

If running in a Windows Container, there is no such directory at all.

If running from within bash on Windows Server, the directory seems to
be fully accessible. (The mechanics of this isn't fully understood, and
it doesn't seem to happen on desktop versions.)

If the directory isn't available with the expected behaviour, mark those
individual tests as unsupported. (The test as a whole is considered to
pass, but the unsupported test is mentioned in a test summary printed on
stdout.)

Differential Revision: https://reviews.llvm.org/D98960