[libc++][nfc] Test std::unique_ptr self move assignment.

The post-conditions for the self move assignment of `std::unique_ptr`
were changed. This requires no implementation changes. A test was added
to validate the new post-conditions.

Addresses
- LWG-3455: Incorrect Postconditions on `unique_ptr` move assignment

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D103764