[libcxx] [test] Fix lexically_normal and lexically_relative_and_proximate for windows

Convert the expected result path to preferred separators, add exceptions
to the test results where needed (due to some cases being interpreted
as a root name).

Differential Revision: https://reviews.llvm.org/D98106

GitOrigin-RevId: d601edf0b053b0df3bea0df16187009b2a0c946b
2 files changed