[libc++] Temporarily force-set the LIBCXX_TEST_CONFIG cache value

This ensures that existing CMake build trees will start using the new
default without having to nuke their build directories.

Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 1013468fe6f513ba6a1770e7043506bd0f026ffe
1 file changed