Simplify data fixtures.

[pp-mo]

This is a follow-on to #5834
Following a discussion on simpler solutions to migrating setUp functions..
This effectively reverts some of the tweaks in the previous : makes fewer diffs from the original unittest form.
May be slightly pointless, but the new way will be easier to follow in the eventual feature merge-back.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.74%. Comparing base (b3eb2d1) to head (b49a508).
Report is 4 commits behind head on FEATURE_pytest_conversion.

Additional details and impacted files

@@                      Coverage Diff                      @@
##           FEATURE_pytest_conversion    #5838      +/-   ##
=============================================================
- Coverage                      89.74%   89.74%   -0.01%     
=============================================================
  Files                             92       92              
  Lines                          22942    22940       -2     
  Branches                        5464     5462       -2     
=============================================================
- Hits                           20590    20588       -2     
  Misses                          1620     1620              
  Partials                         732      732              

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