[jax2tf] Refactor the backwards compatibility tests.
We have two goals:
* separate out the tests that depend on TensorFlow (back_compat_tf_tests.py)
* separate out the boilerplate (back_compat_test_util.py) so that we can use it as a library to set up similar compatibility tests for custom calls that are not used directly by the JAX core.
PiperOrigin-RevId: 542500216
