[Form] Fixed SubmitTypeValidatorExtension not correctly extending BaseValidatorExtension

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8209
| License       | MIT
| Doc PR        | 

Although stated in the documentation is was not actually possible to use "validation_groups" config on a submit form type. Turns out that although a BaseValidatorExtension had be created with the required functionality SubmitTypeValidatorExtension was not yet extending from it.

This pull request corrects this issue, adds tests for SubmitTypeValidatorExtension and as a bonus removes an unused import in FormTypeValidatorExtension.