[libc++] Refactor tests for std::pointer_traits (#66645)

After landing the implementation of LWG3545, I realized that the tests
for std::pointer_traits had become a bit disorganized. This patch is a
NFC that refactors the tests:
- Move compile-only tests to `.compile.pass.cpp` tests
- Re-create the clear distinction between tests for the std::pointer_traits base tempate and for the T* specialization.
- De-duplicate test coverage -- we had a bunch of things that were tested in duplication.