[libc++] [test] Don't nasty-macro `__z` yet; we actually use that one.

This reverts one of the macros just added in D101613, because it turns out
that the <utility> header actually uses the identifiers __x, __y, __z.
We probably *shouldn't* use __z if it's reserved on Windows; but since
it's not causing us any active problem even on Windows, I think this is
the safest way to unbreak the test.