Introduces new testing helpers to simplify testing of declarative validation rules. The new `VerifyValidationEquivalence` and `VerifyUpdateValidationEquivalence` functions reduce boilerplate by encapsulating the logic for:
- Toggling the `DeclarativeValidation` and `DeclarativeValidationTakeover` feature gates.
- Comparing the validation output from the imperative and declarative paths.
The declarative validation tests for CertificateSigningRequest and ReplicationController are updated to use these new, simpler helpers.
This makes a bold assumption: that the errors (count and basic content)
will be the same across versions. If this turns out to be untrue, this
may need to get more sophisticated. It should fail obviously when we
hit that edge.
