Reduce the time spent for the TF slow tests
# What does this PR do?

This PR reduces by half the time spent on running the all the tests (including the slow tests). Here are the time comparison (time recorded on my machine with the models already downloaded):

- albert: from 13mins to 6mins
- bart: from 19mins to 9mins
- bert: from 17mins to 9mins
- blenderbot_small: from 19mins to 9mins
- blenderbot: from 19mins to 9mins
- convbert: from 21mins to 11mins
- ctrl: from 10mins to 7mins
- distilbert: from 13mins to 7mins
- dpr: from 6mins to 3mins
- electra: from 15mins to 7mins
- flaubert: from 13mins to 7mins
- funnel: from 28mins to 13mins
- gpt2: from 8mins to 4mins
- led: from 44mins to 20mins
- longformer: from 1h30mins to 40mins
- lxmert: from 6mins to 3mins
- marian: from 19mins to 9mins
- mbart: from 19mins to 9mins
- mobilebert: from 33mins to 16mins
- mpnet: from 13mins to 7mins
- openai gpt: from 8mins to 4mins
- pegasus: from 19mins to 9mins
- roberta: from 10mins to 6mins
- t5: from 12mins to 7mins
- transfo_xl: from 8mins to 5mins
- xlm: from 13mins to 7mins
- xlnet: from 9mins to 5mins

Total: from 8h5mins to 4h8mins

The total time spent on running the entire tests has been reduced by half by merging three tests about the SavedModel into a single one.
@sgugger Yes, this is exactly that. There was an important overlap across these three tests (all based on creating a saved model and two on testing the output), so merging them was IMO the best way to keep the coverage and reduce the time.

@patrickvonplaten feel free to merge if the PR looks ok for you!