Add explicit jax kernel tests

The recent https://github.com/scikit-hep/awkward/issues/3456, https://github.com/scikit-hep/awkward/issues/3462, https://github.com/scikit-hep/awkward/issues/3463 and https://github.com/scikit-hep/awkward/issues/3465 tell me that we should probably add a folder of full jax kernel tests like we do for the cpu and cuda backends.