Add CEL validation tests for xPolicy resources

[arkodg]

Description:
We've recently started add CEL validations in our APIs e.g. #1961
Would be great to add validation tests so its easier to understand what works and what doesnt
Here's an example of how upstream does it https://github.com/kubernetes-sigs/gateway-api/blob/2b9aeb0c7fa3d558e42074486c230406e7ac80ce/pkg/test/cel/grpcroute_test.go#L416

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[zirain]

we should add valiation for other EG CRDs.