[tests] multiple improvements
This PR:

1. splits the 2 groups of `TrainerIntegrationTest` tests into separate subclasses, since at the moment many run 2x training in `setUp` and then don't use the results - should make things a bit faster, and mainly this removes the weird unexpected logs when debugging tests.
2. introduce, use and document `require_torch_up_to_2_gpus` as we have a bunch of tests that can only run on up to 2 gpus and currently they weren't `skipped`, but hacked to return reporting `passed`!
3. fix `test_resume_training_with_randomness` to use `assertAlmostEqual` so that we get debug data when it fails - and also fix the comment to match the code that this test only works with 0 or 1 gpus - and use the marker.

@sgugger 