[jax2tf] Refactor test_binary_elementwise. (#4043)
* [jax2tf] Reformulate test_binary_elementwise.
* igamma/igammac are unsupported by JAX on (b)float16
* rem/atan2 have missing TF kernels for (b)float16
* nextafter is unsupported for (b)float16
* reactivated bfloat16 tests for the other operations
* Changed error message for rem/atan2 (b)float16 support.
