In order for pytest-xdist to put all the marked tests onto one worker, you need to add --dist loadgroup to the pytest call.
https://pytest-xdist.readthedocs.io/en/latest/distribution.html#running-tests-across-multiple-cpus

Thanks for the tip on --dist loadgroup. @rcomer
However, since that doesn't pause the other worker, I am identifying the specific heavy test causing the starvation so I can lock it exclusively.