[libcxx] [test] Fix string type handling in a few fairly trivial class.path tests

Use string() for convenience for testing where possible, but keep using
native() for move tests where we want to check that no allocations are
made, constructing a reference fs::path::string_type instead.

Use the right value_type in a few places.

Make the synop test check for the right types and for the expected
preferred separator.

Differential Revision: https://reviews.llvm.org/D89537