[libcxx] [test] Fix mismatches between _aligned_malloc and free() on Windows

This allows getting rid of one case of LIBCXX-WINDOWS-FIXME. The fixme
comment was inaccurate; aligned allocation functions are provided these
days, but the test kept failing as it was using mismatched allocation
and free functions.

A similar issue was fixed earlier, in
6596778b. That test was fixed by
overriding the aligned `operator new` too, and returning a dummy fixed
allocation instead. As this test is libcxx specific, it can use the
internal `std::__libcpp_aligned_free()` instead, to match libcxx's
internal aligned `operator new`.

Differential Revision: https://reviews.llvm.org/D118190