test_spill_hysteresis flaky on ubuntu

[fjetter]

This test was already marked as flaky in #5840 for other OS but I've seen a failure on ubuntu as well, see https://github.com/fjetter/distributed/runs/5288437230?check_suite_focus=true

(This is a branch where I am testing things around connect timeouts which should not affect the spill buffer. If it fails on this branch, it is reasonable to believe it will fail on others as well)

Test introduced in #5813

cc @crusaderky