Review skipped and xfail tests

[jacg]

We have accumulated a bunch of skipped and xfailing tests and I suspect that everyone has simply forgotten about them, and ignores them as minor noise in the test output.

We should do a thorough review of each and every one of them to see whether they can/should be fixed or removed or whether they are indicating some significant problems that we are ignoring.

[On a tangentially related issue: we should probably replace @mark.serial with some variation on the theme of @mark.order('last') (from pytest-order, which is being used in Nexus and for which I contributed an package to the official nixpkgs collection.)]