[libc++][nfc] Use TEST_SAFE_STATIC.

This avoids using an libc++ internal macro in our tests.

Reviewed By: #libc, philnik, ldionne

Differential Revision: https://reviews.llvm.org/D118874

NOKEYCHECK=True
GitOrigin-RevId: 5dc1da3e304c9ec22b6e0f880adfe54f8939d8ee
3 files changed