[gator verify] support specifying "oldObject" in test cases

[frankh]

Describe the solution you'd like You should be able to specify oldObject in test cases, exactly like you specify object

I have some policies that have violations depending on changes from the oldObject and these are untestable in gator

Anything else you would like to add: [Miscellaneous information that will assist in solving the issue.]

Environment:

• Gatekeeper version: 3.9.0
• Kubernetes version: (use kubectl version): 1.22.5

[maxsmythe]

This would be fixed by #1819, though we should make it easy to specify/reuse object oldObject across test cases if we can.

[stale[bot]]

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