[libcxx] [test] Fix fs.op.proximate for windows

[libcxx] [test] Fix fs.op.proximate for windows

Simmilar to many other similar path handling tests, convert the
test reference to preferred separators, and ifdef a few test references
that use network root names.

Additionally, generalize code for trimming off the root path for
generating relative_cwd, and for skipping the root name element
in count_path_elems.

Rename one fictive path for consistency with the other test cases,
and add a bunch of more test cases for completeness.

Differential Revision: https://reviews.llvm.org/D98988