POC/TST: dynamic xfail
Motivation: we have a non-trivial number of places where we do `pytest.xfail` within a test (as opposed to via decorator).  This has the downside of always being non-strict, so incorrectly-xfailed tests will never show up as xpassing.

This uses a workaround to dynamically add the pytest.mark.xfail marker within the test rather than raising pytest.xfail directly.

Thoughts?