Migrate away from using JaxTestCase in tests
Migrate away from using JaxTestCase in tests

Why? JaxTestCase is deprecated for use outside the JAX project as of version 0.3.1; see https://jax.readthedocs.io/en/latest/changelog.html#jax-0-3-1-feb-18-2022

PiperOrigin-RevId: 437356983

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