[libc++][test] Use `LIBCPP_STATIC_ASSERT` for `std::__mdspan_detail` (#73436)

Found while running libc++'s test suite against MSVC's STL.

`libcxx/test/std` should be portable, so these lines checking
`std::__mdspan_detail` machinery should be using `LIBCPP_STATIC_ASSERT`.

I checked for other occurrences and these appear to be the only ones
which have appeared since our last libc++ test suite update.