test: add tests for mutation annotations

[chewong]

What this PR does / why we need it:

Adding test cases and documentations for mutation annotations gatekeeper.sh/mutation-id and gatekeeper.sh/mutations.

Which issue(s) this PR fixes (optional, using fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when the PR gets merged):
Fixes #1561

Special notes for your reviewer:

[codecov-commenter]

Codecov Report

Merging #1846 (03c3ba5) into master (b36e855) will decrease coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1846      +/-   ##
==========================================
- Coverage   52.27%   52.12%   -0.16%     
==========================================
  Files         100      100              
  Lines        8956     8956              
==========================================
- Hits         4682     4668      -14     
- Misses       3900     3910      +10     
- Partials      374      378       +4     

Flag	Coverage Δ
unittests	52.12% <ø> (-0.16%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/readiness/list.go	79.41% <0.00%> (-11.77%)	⬇️
...onstrainttemplate/constrainttemplate_controller.go	57.93% <0.00%> (-1.69%)	⬇️
pkg/readiness/object_tracker.go	82.91% <0.00%> (-1.07%)	⬇️

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[sozercan]

LGTM

[maxsmythe]

LGTM!