[libcxx] [test] Fix path.decompose for windows

Add ifdefs to the test reference tables for cases where paths are
interpreted differently (paths that contain a root name).

Fix test assumptions regarding has_root_name() and is_absolute() and
add logic to verify the results of is_absolute() for the test cases in
the table.

Also add a testcase for the path "//net/", which seemed like an
omission.

Differential Revision: https://reviews.llvm.org/D89943

GitOrigin-RevId: 1adaf48d231d1dfbd7ff575e2d03c8b01a512fe7
1 file changed