[CIs] report slow tests add --durations=0 to some pytest jobs
It appears that over the last 10 days or so the CIs have gone 4-5 times slower. From 2-3 minutes to 10-11 minutes for `run_tests_torch`.

This PR adds a report, generated by `pytest`, that lists the slowest tests so that we could quickly detect such regressions in our test suite's performance.

As suggested by @sshleifer, pytest can do the diagnostics using `--durations=` flag. I'm not sure what the best number to set it to - let's try with 50:

I propose adding this to:
* `run_tests_torch_and_tf` job - as it runs all non-slow tests in "real-time"
* all scheduled jobs that run slow tests

**edit**: Several fixes have been merged since this slowdown was found, so we are much better off and there is an effort to perform a major clean up here https://github.com/huggingface/transformers/pull/7895 so support this effort let's start with reporting the runtime of all tests, that is `--durations=0` (`pytest` currently doesn't have an option to report only tests slower than a certain run time) and once the clean up has been done, then we dial down to `--durations=50` to only monitor any new outliers.

@sshleifer, @sgugger, @LysandreJik 