[libc++] Remove MSVC tests checked into the libc++ test suite

We should strive to have our own tests, except when there is overwhelming
value in using another standard library's existing tests. The reason is
that it ensures that implementations don't all start relying on the same
interpretation of the Standard.

The unique_ptr tests did not add any test coverage AFAICT, and the
forward_like tests were moved to the style used everywhere in the
libc++ test suite.

Note that I got to this because this actually broke a downstream
configuration where we use -ffreestanding. The signature of main()
was not consistent with the signature we (need to) use everywhere
in the test suite.

Differential Revision: https://reviews.llvm.org/D134767