TST: Disallow pytest.xfail
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="..."))

 - [x] 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:

 - [x] indexes/test_common
 - [x] pandas/conftest
 - [x] indexes/test_setops
 - [x] indexing/test_coercion
 - [x] series/indexing/test_setitem
 - [x] series/test_arithmetic
 - [x] series/indexing/test_where
 - [x] tseries/offsets