Make XPASSing tests fail the CI so we remove XFAIL marks

[michaelosthege]

On the way to the v4.0.0 release we're striving to get our test suite back ✔.

We currently have a lot of tests that are already passing again, but are still marked as XFAIL.

Example: https://github.com/pymc-devs/pymc3/runs/2754411365#step:7:33

70f2afa was supposed to make XPASSing tests fail the CI so we detect them and remove the XFAIL.
But it looks like this setting does not have an effect.