Add CEL tests for `SnippetsFilter` CRD

[shaun-nx]

Overview

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

UACs

• Add test for the context CEL validation

context validation:

x-kubernetes-validations:
- message: Only one snippet allowed per context
  rule: self.all(s1, self.exists_one(s2, s1.context == s2.context))

[nginx-bot]

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