increase coverage for accelerators [WIP]

[awaelchli]

What does this PR do?

Draft PR, need to deactivate ignore rules first before I can see the uncovered lines.
Will update this PR with new tests for accelerator api.

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[codecov]

Codecov Report

Merging #6102 (01f26b4) into master (ae6ce17) will decrease coverage by 1%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #6102    +/-   ##
=======================================
- Coverage      91%     90%    -1%     
=======================================
  Files         160     161     +1     
  Lines       11405   11442    +37     
=======================================
- Hits        10417   10316   -101     
- Misses        988    1126   +138     

[Borda]

@awaelchli how is it going here, still WIP or ready to go? 🐰