TST: Disallow pytest.xfail

[rhshadrach]

pytest.xfail will immediately stop a test, not checking if it would have otherwise succeeded. The request fixture can used to run a test and only pass if the test does indeed fail:

request.node.add_marker(pytest.mark.xfail(reason="..."))

• Remove all occurrences of pytest.xfail
• Update docs
• Add pre-commit hook

PR #38901 handles many of the current occurrences. The remaining ones are in:

• indexes/test_common
• pandas/conftest
• indexes/test_setops
• indexing/test_coercion
• series/indexing/test_setitem
• series/test_arithmetic
• series/indexing/test_where
• tseries/offsets