[libcxx] [test] Clarify and improve consistency in lexically_relative_and_proximate.pass.cpp. NFC.

Use "expect" instead of "output" for generating "proximate_expected",
pass the arguments to PathEq in the same order as above, rename the
"proximate_expected" variable to be consistent with the naming of the
earlier "expect", use .empty() instead of .native().empty().

Differential Revision: https://reviews.llvm.org/D98127