Add missing tokenizer tests - RemBert
# What does this PR do?

Added tokenizer tests for Rembert. I took reference from `test_tokenization_camembert.py`

@SaulLu @LysandreJik 
_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks for adding this! Can you first rebase your branch on main? TensorFlow new release broke a lot of things so tests won't pass unless you do this :-)<|||||>Looks like something went wrong and GitHub adds a lot of diff. If force-pushing doesn't solve the issue, you might need to close this PR and open a fresh one.