[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.
- LWG-3455: Incorrect Postconditions on unique_ptr move assignment
Reviewed By: #libc, ldionne
Differential Revision: https://reviews.llvm.org/D103764