[jax2tf] Build a primitive harness for test_type_promotion. (#4279)
* [jax2tf] Build a primitive harness for test_type_promotion.
We were previously generating the cases using `jtu.cases_from_list`,
which by default dropped 2 test cases (JAX_NUM_GENERATED_CASES=10,
number of generated cases = 12).
* [jax2tf] Fix the generated test cases for test_type_promotion.
