XYZDelaunayGenerator tests often time out on valgrind

[loganharbour]

Motivation

The XYZDelaunayGenerator tests often time out on valgrind. We're not sure what the issue is yet.

Design

Skip the tests for now to avoid holding up CI, but keep this issue open to investigate.

Impact

Reduced testing for now.

[roystgnr]

For what it's worth, I can replicate the valgrind timeouts locally, on my usual cobbled-together-stack Ubuntu builds, so whatever is triggering them isn't a CI environment quirk. I haven't been able to figure out much more than that, though.