ament_cmake exports a dependency on ament_cmake_test which adds the option BUILD_TESTING. This matches the CTest module which "automatically creates a BUILD_TESTING option that selects whether to enable testing support (ON by default)".
If ament_cmake_test isn't required, then calling find_package() on the individual ament_cmake_* packages needed by a project is the way to go. If ament_cmake_test is needed, the BUILD_TESTING option can be turned off on the command line with the cmake argument -DBUILD_TESTING=OFF. If you're using colcon, use colcon build --cmake-args -DBUILD_TESTING=OFF.