Default `set_up_ament` to `True` for `ros2_cpp_test`, `ros2_py_test`

[lalten]

I believe this is a reasonable default because it shouldn't affect the observable behavior of existing tests, but will (since https://github.com/mvukov/rules_ros2/pull/100) save new users from some debugging when they try to figure out what is causing the rcutils_expand_user failed, Failed to get logging directory errors in their tests that spin up the RCL.

[mvukov]

@ahans PTAL.