* Centralize mocks initialization in ILM steps tests
This change centralizes initialization of `Client`, `AdminClient`
and `IndicesAdminClient` for all classes extending `AbstractStepTestCase`.
This removes a lot of code duplication and make it easier to write tests.
This also removes need for `AsyncActionStep#setClient`
* Unused imports removed
* Added missed tests
* Fix OpenFollowerIndexStepTests