More Keras 2 tests that are skipped for Keras 1
More Keras 2 tests that are skipped for Keras 1

- Added `skipIf` whenever there is a mark for keras 2
- Also avoids explicit argument in `reset_states` (API change occurred at
  2.0.4 renaming the argument `states_value` to `states`; this is the lowest
  we support, so it does not affect us, but it also doesn't hurt)
- Removed individual `pytest.mark.keras2` when the whole class is marked

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