TF: tests for (de)serializable models with resized tokens
# What does this PR do?

Since I'm touching the part of our code that handles resizing of token embeddings, I decided to boost our test suite there.

This PR adds two tests regarding resizing token embeddings, on the TF side:
1. Tests that we can resize the embeddings of a model, save it, and then restore it (with the resized embeddings), while keeping the same outputs for a given input in the resized range;
2. Tests that passing inputs outside the vocabulary triggers an exception -- surprisingly, TF doesn't do this check on GPU, which means that a user can resize the embeddings incorrectly and run forward passes (inference or training) with incorrect numerical results, but no exceptions.

⚠️ The tests added above do not pass in all cases, and fixes will be arriving over subsequent PRs. To ensure it doesn't manifest in our push CI, the following touches were added -- alternative suggestions are welcome!
- Test 1. is failing for several models, so a `@slow` decorator was added to keep track of the failures while allowing the push CI to pass. It seemed more sensible to me than to add `skip` in all failing cases 🤔 
- Test 2. Fails for all models with embeddings except BART, on GPU. Our push CI doesn't use GPU, so it is not impacted.
_The documentation is not available anymore as the PR was closed or merged._