Refactor admin tests

[florimondmanca]

Prompted by #102 (comment)

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@7136f1a). Click here to learn what that means.
The diff coverage is 95.45%.

@@            Coverage Diff            @@
##             master     #108   +/-   ##
=========================================
  Coverage          ?   96.44%           
=========================================
  Files             ?       22           
  Lines             ?      478           
  Branches          ?        0           
=========================================
  Hits              ?      461           
  Misses            ?       17           
  Partials          ?        0

Impacted Files	Coverage Δ
tests/conftest.py	98.03% <ø> (ø)
tests/test_permissions_custom.py	100% <100%> (ø)
tests/test_permissions_combination.py	100% <100%> (ø)
tests/utils.py	100% <100%> (ø)
tests/test_permissions.py	98.36% <100%> (ø)
src/rest_framework_api_key/admin.py	76.92% <66.66%> (ø)

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