Split backend tests into different workflows

[gdalle]

DI tests

• Make tests depend on an environment variable ENV["JULIA_DI_TEST_GROUP"] with the following options:
  • "All" (only useful locally)
  • "Formalities" => Aqua, JET, JuliaFormatter
  • "Internals" => various checks that don't require AD backends
  • "Single/Backend1" or "Double/Backend1-Backend2" => install the required packages and include the appropriate tests

CI

• Set the variable ENV["JULIA_DI_TEST_GROUP"] with a workflow matrix before the tests
• Exclude 1.6-incompatible backends from the matrix

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.03%. Comparing base (3634522) to head (3357728).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #269   +/-   ##
=======================================
  Coverage   96.03%   96.03%           
=======================================
  Files          75       75           
  Lines        3908     3908           
=======================================
  Hits         3753     3753           
  Misses        155      155           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.