When too many tests are run, phpunit will segfault after it finishes running the tests. It might be related to the total number of asserts, or memory allocated, but mediawiki core is close to some limit, where patchsets with a lot of tests can trigger this condition (see https://gerrit.wikimedia.org/r/#/c/77645/, patchsets 23-25).
I was able to reproduce the segfault in a patch that only contained noop tests too. Somewhere between 150-175 tests that just assertFalse(false) will trigger it. See https://gerrit.wikimedia.org/r/#/c/118573/ for plenty of patchsets that reproduce the issue.
I'm afraid if we add too many more tests to core, we're going to quickly start hitting this more frequently, so it would be good to try and get to the root of it.