[MRG] reorganize tests per backends

[rflamary]

Types of changes

Motivation and context / Related issue

How has this been tested (if it applies)

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.

[codecov]

Codecov Report

❌ Patch coverage is 55.55556% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.81%. Comparing base (6c98fac) to head (837d2b4).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #828      +/-   ##
==========================================
+ Coverage   96.80%   96.81%   +0.01%     
==========================================
  Files         127      126       -1     
  Lines       25136    25104      -32     
==========================================
- Hits        24333    24305      -28     
+ Misses        803      799       -4     

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