Add unit tests for applierGroup.Get

[siddhiguptas]

What this PR does / why we need it:
This PR resolves the TODO: Add test for this applierGroup function comment in pkg/app/piped/executor/kubernetes/applier_group.go.

It introduces a table-driven test suite TestApplierGroupGet in applier_group_test.go to cover the various matching and error-handling conditions of the Kubernetes resource applier routing logic.

Which issue(s) this PR fixes:
Fixes #7003

Does this PR introduce a user-facing change?:

• How are users affected by this change: None
• Is this breaking change: No
• How to migrate (if breaking change): N/A

[github-actions]

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