Add unit-test to ensure all validator subclasses are annotated with @GtfsValidator

[bdferris-v2]

Describe the problem

When implementing a new validation rule, I forgot to annotate my validator with the @GtfsValidator annotation. This meant it wasn't picked up automatically by ValidatorLoader and not run as part of the default validation set, even though individual unit-tests for this validator ran successfully.

Proposed solution

Add a unit-test that checks that any subclass of SingleEntityValidator or FileValidator is also annotated with @GtfsValidator.

Alternatives you've considered

We could alternatively rewrite ValidatorLoader to ignore the @GtfsValidator annotation and instead scan for subclasses directly. I'll admit I wasn't around for the original design choice, but I could see an argument for having a single annotation type across all validators, regardless of subclass, especially if the validator subclasses grows in the future. Either way, given current staffing on the project, I'm in favor of keeping the status quo for now.

Additional context

No response