Fix GoogleTest 1.14.0 import

• Prune headers and tests no longer installed after the upgrade.
• Remove GoogleTest-related files when MK_GOOGLETEST == no.
• Disable -Werror with gcc to unbreak the gcc12 CI run with lib/googletest. Any issues found by g++ will be filed upstream and hopefully resolved in a future version.
• Remove clang -Werror issues which are resolved in version 1.14.0 to avoid masking valid issues.

MFC after: 1 week
MFC with: 28f6c2f292806bf31230a959bc4b19d7081669a7