Fix TestChangeDefaultDomain flake

[dprotaso]

Was able to repro this on my machine

The test needs to eventually wait for the certificate to be deleted.

Fixes: #15364

/assign @EraKin575

[knative-prow]

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In response to this:

Was able to repro this on my machine

The test needs to eventually wait for the certificate to be deleted.

Fixes: #15364

/assign @EraKin575

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.51%. Comparing base (ac7f585) to head (5f0bf0c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15380      +/-   ##
==========================================
- Coverage   84.58%   84.51%   -0.08%     
==========================================
  Files         219      219              
  Lines       13587    13587              
==========================================
- Hits        11493    11483      -10     
- Misses       1726     1733       +7     
- Partials      368      371       +3     

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[ReToCode]

/lgtm

[knative-prow]

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