[libc++] Mark tests that use check_assertion.h as requiring unix headers

On targets without unistd.h or sys/wait.h (such as bare metal targets)
any test that uses check_assertion.h will fail, so add
REQUIRES: has-unix-headers to them and autodetect whether we have
these headers or not.

These tests currently have unsupported on windows, but that's exactly
because windows doesn't have these headers so we can remove the
specific check for windows.

Differential Revision: https://reviews.llvm.org/D124623