Add CEL validation tests for `AuthenticationFilter` CRD

[shaun-nx]

Overview

As a user of NGF, I would like to add CEL tests for the AuthenticationFilter CR so that I can be confident that the CEL validation works as I expect

Acceptance

• Add new test file to tests/cel directory for the AuthenticationFilter

Validations to test

1. for type=Basic, spec.basic must be set

Example yaml that will fail validation:

apiVersion: gateway.nginx.org/v1alpha1
kind: AuthenticationFilter
metadata:
  name: basic-auth
spec:
  type: Basic
  # spec.basic is missing