[CI-Test] Fixes but also skips the mT5 tests
# What does this PR do?
Silences the `MT5` tests. They are irrelevant as the model behind is T5, which function well. 
I want to delete everything but prefer to ask if there is a reason why we have these? 
cc @sgugger 
My opinion is that we should have test file(s) for each model type we have. MT5 has its own modeling file and model type, so we should keep it.
<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>mT5 does not really have its own modeling file since it's just T5. I'm happy if the tests for that model are contained to a slow integration test to not bloat the CI, like what is done for similar models (Camembert for instance).<|||||>I am OK with the decision, but just want to point out that we will lose these models for tiny model creation, which is required for ONNX testing and pipeline testing (in the future). I just want every party involved agree this.

cc @LysandreJik, @Narsil  and @lewtun <|||||>Ok with the decision as well!