🧪 Unmeasure coverage in tests expected to fail (#15512)

These tests are known to only be executed partially or not at all. So
we always get incomplete, missing, and sometimes flaky, coverage in
the test functions that are expected to fail.

This change updates the ``coverage.py`` config to prevent said tests
from influencing the coverage level measurement.

Ref https://github.com/pytest-dev/pytest/pull/12531

.coveragerc

@@ -23,6 +23,8 @@ exclude_lines =
     if 0:
     if __name__ == .__main__.:
 
+    ^\s*@pytest\.mark\.xfail
+
 ignore_errors = True
 
 [xml]