Implement "check" target in CMake build

The CMake build currently has a dummy "check" target that exists only to allow the "distcheck" sequence to complete successfully. The "check" target needs to be updated to properly invoke test-libmongoc (same as the autotools "check" target). An consideration is maybe we can implement "make test" in cmake to run test-libmongoc with MONGOC_TEST_LIVE=off (unless the outside environment explicitly set MONGOC_TEST_LIVE=on) then have "make check" depend on that "make test" target. The "make distcheck" already depends on "make check".