minor #38932 [Validator] Merge RecursiveValidatorTest with its parents (derrabus)

This PR was merged into the 4.4 branch.

Discussion
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[Validator] Merge RecursiveValidatorTest with its parents

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | N/A
| License       | MIT
| Doc PR        | N/A

`RecursiveValidatorTest` as two abstract parent classes that to my understanding exist mainly for historic reasons: We used to have different validator implementations and those abstract classes contained some shared test cases. This is not the case anymore, which leaves `RecursiveValidatorTest` as the only child class of those two abstract classes.

This PR suggests to merge the three classes into one.

I've executed the test suite before and after the change and PHPUnit reported the same number of assertions and test cases.

I've also checked how the merge to 5.1 and 5.x would look like. The merge to 5.1 works without conflicts. From 5.1 to 5.x, two additional test cases have to be moved down and the reference to a few fixture classes needs to be adjusted.

Commits
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6822774d37 [Validator] Merge RecursiveValidatorTest with its parents.

Author: fabpot