[No merge] TF integration testing
Adds integrationt tests for BERT, ELECTRA and Longformer to ensure that PR such as https://github.com/huggingface/transformers/pull/7605 do not impact the current state of models. RoBERTa not done because it's already done.

Patches a bug with the `ElectraForPreTraining` when batch size = 1
@LysandreJik Is this PR done with your last changes in the tests?<|||||>Ah, I had forgotten about this. I'll rebase and ping you for review
<|||||>should be good for review @jplu <|||||>They're not the same as they don't rely on the full checkpoints but on some random tiny ones, to make the CI faster.
It does test every same aspect, however: the weights loading, the full inference, the expected results.