[libc++][spaceship[NFC] P1614R2 `operator<=>` moves misplaced tests to correct location

P1614R2:

- Moved misplaced tests from `libcxx/test/libcxx` to `libcxx/test/std`
- Updated status page `SpaceshipProjects.csv`: `operator<=>` implementation status

Related docs:

- https://eel.is/c++draft/
- https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1614r2.html

Reviewed By: #libc, philnik

Differential Revision: https://reviews.llvm.org/D146902