infrastructure/testdriver/click_iframe_crossorigin.sub.html can fail in Firefox

[foolip]

This happened in https://github.com/web-platform-tests/wpt/pull/45985/checks?check_run_id=24417704584

From the logs:

Unexpected Results
------------------
/infrastructure/testdriver/click_iframe_crossorigin.sub.html
  FAIL TestDriver click on a document in an iframe - uncaught exception: Error: assert_unreached: click failed Reached unreachable code
@http://web-platform.test:8000/infrastructure/testdriver/click_iframe_crossorigin.sub.html:15:25

This seems similar to #26295.

cc @jgraham

[jgraham]

Seems to be totally stable on Linux and in Safari on macOS: https://wpt.fyi/results/infrastructure/testdriver/click_iframe_crossorigin.sub.html?label=master&label=experimental&aligned&q=infrastructure%2Ftestdriver%2F

I'm not seeing where there could be a race condition in the test, and I don't have an easy way to debug locally.