[Test] Refactor inject_webhook_test.go according to the Developer Guide

[Electronic-Waste]

What this PR does / why we need it:

I refactored /pkg/webhook/v1beta1/pod/inject_webhook_test.go with rule described in Developer Guide:

1. Use cmp.Diff instead of reflect.Equal, to provide useful comparisons.
2. Define test cases as maps instead of slices to avoid dependencies on the running order. Map key should be equal to the test case name.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #2388

Checklist:

• Docs included if any changes are user facing

[Electronic-Waste]

I've added the missing UT which was deleted by accident now. PTAL👀 @tenzen-y if you have time.

[tenzen-y]

Thanks!
/lgtm
/approve

[google-oss-prow]

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