User activation tests are flaky

[CanadaHonk]

User activation tests (/html/user-activation/...) are still being newly flaky for Gecko (recently implemented) and seemingly Chromium too still. Needs more investigation (happy to assist). See also past #36221.

Engine bugs on flakiness:

• Gecko (new)
• Chromium #1 (inactive), Chromium #2 (inactive)
• WebKit (outdated?)

wpt.fyi results. Notes:

• Gecko times out on some cross-origin tests due to an unrelated cross-process webdriver bug, but they are still flaky ignoring this.
• Chromium essentially perma-fail navigation state reset tests for wpt.fyi afaik.

[gsnedders]

• WebKit (outdated?)

Seems still flaky on the results dashboard: https://results.webkit.org/?suite=layout-tests&suite=layout-tests&suite=layout-tests&suite=layout-tests&test=imported%2Fw3c%2Fweb-platform-tests%2Fhtml%2Fuser-activation%2Fconsumption-crossorigin.sub.html&test=imported%2Fw3c%2Fweb-platform-tests%2Fhtml%2Fuser-activation%2Fconsumption-crossorigin.sub.tentative.html&test=imported%2Fw3c%2Fweb-platform-tests%2Fhtml%2Fuser-activation%2Fconsumption-sameorigin.html&test=imported%2Fw3c%2Fweb-platform-tests%2Fhtml%2Fuser-activation%2Fconsumption-sameorigin.tentative.html