[#34] Add test for certificate deletion

[SilviaAmAm]

Fixes #34

[codecov]

Codecov Report

Merging #37 (1af56da) into master (aedcd67) will increase coverage by 1.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #37      +/-   ##
==========================================
+ Coverage   45.01%   46.16%   +1.15%     
==========================================
  Files          36       36              
  Lines        1304     1304              
  Branches      222      222              
==========================================
+ Hits          587      602      +15     
+ Misses        683      668      -15     
  Partials       34       34              

Impacted Files	Coverage Δ
zgw_consumers/mixins.py	90.90% <0.00%> (+45.45%)	⬆️

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