Revisit unittest handling

[blueyed]

While looking into #710.

[codecov-io]

Codecov Report

Merging #740 into master will decrease coverage by 0.29%.
The diff coverage is 89.47%.

@@           Coverage Diff            @@
##           master    #740     +/-   ##
========================================
- Coverage    97.9%   97.6%   -0.3%     
========================================
  Files          32      32             
  Lines        1863    1881     +18     
  Branches      147     147             
========================================
+ Hits         1824    1836     +12     
- Misses         26      32      +6     
  Partials       13      13

Impacted Files	Coverage Δ
pytest_django/plugin.py	94.01% <89.47%> (-0.02%)	⬇️
tests/test_db_setup.py	95.23% <0%> (-4.77%)	⬇️

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[blueyed]

Only not skipped is covered here. But also previously.