Benchmark model tests: fix gradients checks

[dweindl]

• Fix gradient checks. Previously, failing test would not have been reported (see Reduce tolerances in benchmark model gradient checks #2511).
  Quite some tests are now skipped, but better to test some than none... To be re-enabled later.
• Sort test cases to avoid issues with pytest-xdist.
• Refactor

Requires ICB-DCM/fiddy#37

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.73%. Comparing base (be02a6e) to head (7f5e173).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2512      +/-   ##
===========================================
+ Coverage    77.66%   77.73%   +0.07%     
===========================================
  Files          324      324              
  Lines        20951    20951              
  Branches      1451     1451              
===========================================
+ Hits         16271    16287      +16     
+ Misses        4677     4661      -16     
  Partials         3        3              

Flag	Coverage Δ
cpp	73.48% <ø> (+0.07%)	⬆️
cpp_python	34.11% <ø> (ø)
petab	36.62% <ø> (ø)
python	72.29% <ø> (+0.07%)	⬆️

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see 4 files with indirect coverage changes