Validate xunit2 files against the schema

[nicoddemus]

Fix #5095

[codecov]

Codecov Report

Merging #5605 into features will decrease coverage by 39.98%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##           features    #5605       +/-   ##
=============================================
- Coverage     96.13%   56.15%   -39.99%     
=============================================
  Files           117       56       -61     
  Lines         25884    10915    -14969     
  Branches       2496     2037      -459     
=============================================
- Hits          24883     6129    -18754     
- Misses          694     4108     +3414     
- Partials        307      678      +371

Impacted Files	Coverage Δ
src/_pytest/helpconfig.py	23.96% <0%> (-73.56%)	⬇️
src/_pytest/unittest.py	24.21% <0%> (-71.06%)	⬇️
src/_pytest/junitxml.py	25.44% <0%> (-70.42%)	⬇️
src/_pytest/mark/legacy.py	29.16% <0%> (-68.75%)	⬇️
src/_pytest/pastebin.py	19.29% <0%> (-68.43%)	⬇️
src/_pytest/debugging.py	21.98% <0%> (-68.07%)	⬇️
src/_pytest/setuponly.py	29.78% <0%> (-65.96%)	⬇️
src/_pytest/mark/evaluate.py	32.22% <0%> (-64.45%)	⬇️
src/_pytest/resultlog.py	23.88% <0%> (-62.69%)	⬇️
src/_pytest/skipping.py	34.37% <0%> (-61.46%)	⬇️
... and 98 more

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[nicoddemus]

Updated.

Any reviewers? From my POV after this PR gets merged we can release 5.1.

[asottile]

lg2m!

[nicoddemus]

Rebased

[FRidh]

Would it be possible to skip tests/files that require xmlschema when it is not present in the environment? The reason I am asking is because pytest is a test dependency of xmlschema.
This is already done for pexpect and xdist.

In Nixpkgs that means we now have to disable tests of xmlschema because we need to build pytest first. Clearly we have to improve our situation of circular test dependencies here (there is also elementpath to consider), however, that is not trivial. For now we will disable these tests. when running the pytest suite.

[nicoddemus]

@FRidh sure, I don't see any harm on doing so. Would be glad to review/merge a PR in that direction. 👍