Including the `CTest` modules ensures that the `BUILD_TESTING` option is
defined and automatically calls `enable_testing()` if needed. It does not
change the default or introduce any dependencies on its own.
This follows what Abseil already does in their top-level `CMakeLists.txt`.
PiperOrigin-RevId: 443305646
Change-Id: If067c17470f497437c7748aab4aab5227c26e84f
