[soft navs] Better timeout behavior for WPT almost-soft-navigation.html

Previously, there wasn't an individual timeout on the subtests for this
smoke test. I added a bit of code to the test helper to make this
easy-ish to express. In order to get a decent timeout behavior, we need
to start the timer / racing only after the click, so the creation of the
promise and the awaiting with timeout must be separate, I think, so that
soft nav events get collected from the point of creating the original
promise, and the clock starts ticking later. Appears to work pretty well
in local testing (the test runs really fast).

Change-Id: Ie6570d41e3b7227f0db384b3a566f0d0791fe3ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6630332
Commit-Queue: Johannes Henkel <johannes@chromium.org>
Reviewed-by: Scott Haseley <shaseley@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1471438}