Fix IT to use proper superclass and avoid using of different junit versions

[Yury-Fridlyand]

Is your feature request related to a problem?

1. 80% of IT uses junit4 and 20% uses junit5 (jupiter).
2. All IT classes should derive from OpenSearchIntegTestCase, but they derive from OpenSearchRestTestCase now (ref).
3. Any custom RestIntegTestTask can discover only junit5 test cases now.

What solution would you like?

1. P1. Eliminate junit5 in IT.
2. P2. Rework IT to inherit OpenSearchIntegTestCase.
3. P3. Identify and fix other issues with test discovery.

What alternatives have you considered?

Add a dev doc and make all tests compatible with junit4 and junit5 both.

Do you have any additional context?

Note: we can't upgrade to junit5, as reta said on slack:

OpenSearch test scaffolding is build on top of Apache Lucene and randomizedtesting , which both are using JUnit 4, see please randomizedtesting/randomizedtesting#256