chore: revamp testing CI with dedicated environments

[gdalle]

• Split DI's testing CI between Core and Backends
• Introduce a dedicated environment for each backend, thus avoiding Pkg operations inside the test suite
• Remove Diffractor tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.22%. Comparing base (e40ad46) to head (60ef27b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #926      +/-   ##
==========================================
+ Coverage   98.11%   98.22%   +0.11%     
==========================================
  Files         133      133              
  Lines        7971     7959      -12     
==========================================
- Hits         7821     7818       -3     
+ Misses        150      141       -9     

Flag	Coverage Δ
DI	99.00% <100.00%> (+0.15%)	⬆️
DIT	96.19% <ø> (ø)

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