Disable flaky tests
Blocking merge on
https://gerrit.wikimedia.org/r/c/mediawiki/extensions/GrowthExperiments/+/512926#message-8b7bc8f3e6945f3f12881ecea5c6db160f2353a4,
see also https://integration.wikimedia.org/ci/job/quibble-vendor-mysql-hhvm-docker/50925/console
Bug: T224546
Change-Id: I836e515afbefdb1d35cd70a8fd33b0ba0e3eb902