separate test workflow for datasets

[ArneBinder]

This PR outsources tests to new test_datasets.yaml GitHub CI workflow. This is the first approach to handle #112.

The following items in the workflow configs are adjusted:

• on.(push|pull_request).paths(-ignore): only execute the workflow if the respective files did change
• add --ignore=tests/dataset_builders / tests/dataset_builders to pytest arguments: only execute the respective tests
• set --cov=src / --cov: only calculate the coverage for the respective files
• coverage.xml / coverage_datasets.xml to save and upload the coverage report

Notes:

• this also adds new tests for the BratBuilder. Until now, this was covered by the Brat dataset tests, but since these are now separated, we also need explicit tests for the builder.
• this also implements BratBuilder._generate_example() to make it "API-complete"

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.42%. Comparing base (16410c9) to head (195642e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #117      +/-   ##
==========================================
- Coverage   95.47%   94.42%   -1.05%     
==========================================
  Files          23       23              
  Lines        1502     1507       +5     
==========================================
- Hits         1434     1423      -11     
- Misses         68       84      +16     

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