Treat XPASS status as a test failure

This causes full "failure" output to be printed when a test marked
xfail unexpectedly passes, and for the test run to be considered
failing as a result.

The immediate purpose of this change is to facilitate efficient
identification of recently introduced wrong or overbroad xfail
markings.

This behavior may eventually become the pytest default (see gitpython-developers#1728
and references therein), and this could be retained even after the
current xpassing tests are investigated, to facilitate timely
detection of tests marked xfail of code that is newly working.

(Individual tests decorated `@pytest.mark.xfail` can still be
allowed to unexpectedly pass without it being treated like a test
failure, by passing strict=False explicitly.)

pyproject.toml

@@ -8,6 +8,7 @@ filterwarnings = "ignore::DeprecationWarning"
 python_files = "test_*.py"
 tmp_path_retention_policy = "failed"
 testpaths = "test"  # Space separated list of paths from root e.g test tests doc/testing.
+xfail_strict = true  # Treat the XPASS status as a test failure (unless strict=False is passed).
 # --cov   coverage
 # --cov-report term  # send report to terminal term-missing -> terminal with line numbers  html  xml
 # --cov-report term-missing # to terminal with line numbers