pytestplugin: introduce @pytest.mark.lg_xfail_feature

[Bastian-Krause]

Description
@pytest.mark.lg_xfail_feature() is meant to mark a test as "xfail" if the target or global environment contains the specified feature flag. There is no other sane way of testing for features in pytest's collection phase. Marking a test as xfail later is unsupported in pytest. So a pytest.mark decorator is necessary.

Imagine two targets contain the camera feature flag. One of them has the additional special-camera-2000 feature flag. The other has the additional special-camera-3000 feature flag. Due to a known bug on special-camera-3000, the test is expected to fail. The test can be marked as xfail for that feature:

import pytest

@pytest.mark.lg_feature("camera"])
@pytest.mark.lg_xfail_feature(
    "special-camera-3000",
    reason="known bug xy on special-camera-3000",
    raises=AssertionError,
    strict=True,
)
def test_camera(target):
    pass

While at it, improve error messages, skip reason and documentation of @pytest.mark.lg_feature() and feature flags. Especially the documentation changes make it easier to add documentation for the new @pytest.mark.lg_xfail_feature().

Checklist

• Documentation for the feature
• Tests for the feature
• Add a section on how to use the feature to doc/usage.rst
• PR has been tested

[codecov]

Codecov Report

❌ Patch coverage is 0% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.3%. Comparing base (2daa13d) to head (edee111).
⚠️ Report is 2 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
labgrid/pytestplugin/hooks.py	0.0%	23 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1751     +/-   ##
========================================
- Coverage    45.4%   45.3%   -0.1%     
========================================
  Files         172     172             
  Lines       13503   13516     +13     
========================================
  Hits         6131    6131             
- Misses       7372    7385     +13     

Flag	Coverage Δ
3.10	45.3% <0.0%> (-0.1%)	⬇️
3.11	45.3% <0.0%> (-0.1%)	⬇️
3.12	45.3% <0.0%> (-0.1%)	⬇️
3.13	45.3% <0.0%> (-0.1%)	⬇️
3.9	45.4% <0.0%> (-0.1%)	⬇️

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[Emantor]

Overall looks good, some smaller suggestions.

[Bastian-Krause]

Changes:

• raise pytest.UsageError for unexpected marker args and add expected function signatures to error message (for both @pytest.mark.lg_feature and @pytest.mark.lg_xfail_feature)
• add/use env_feature_config factory fixture for env config creation to tests/test_flags.py
• add tests for pytest.UsageErrors (for both @pytest.mark.lg_feature and @pytest.mark.lg_xfail_feature)