Fixes https://github.com/spring-projects/spring-integration/issues/3839
* Create a new base interface for Mongo container-based tests
* Migrate all tests to JUnit5
* Remove obsolete classes related to JUnit4 Mongo rule
* Fix code style & readability here and there, a few Sonar issues
* Fix failing test `validateWithConfiguredPollerFlow`
* * This test was failing with Mongo error
`$and/$or/$nor must be a nonempty array`. The cause was too small
polling ratio of the update query, the consecutive read queries
failed as no 'Oleg' document was in collection after ~100 ms.
Couple of changes after the pull request review:
* Get back the blank lines in test classes
* Get back the blank lines for some inner classes
* Eliminate bad renaming consequences
* Remove a couple of garbage todos
* Couple of places with code cleanup
50769d0350