Improve PolicyException validation test coverage

[karthikmanam]

Summary

The current PolicyException validation tests in api/policies.kyverno.io/v1beta1/policy_exception_test.go leave some validation paths untested.

Specifically:

• TestCELPolicyException_GetKind contains duplicate subtest names (not set).
• TestCELPolicyExceptionSpec_Validate includes a // TODO: Add test cases. comment.
• Validation cases for:
  • a policy reference missing both name and kind
  • multiple invalid policy references with indexed field paths

are currently not covered.

Proposed change

• Rename the duplicate subtest for better test output.
• Add the missing validation test cases.
• Remove the existing TODO after adding the coverage.