[Interop] Make event coalescing test optional as per the spec.
PointerEvent coalescing under load is an optional spec requirement,
so a browser is in fact free not to coalesce any event even when the
main thread is swamped.
https://w3c.github.io/pointerevents/#coalesced-events
This CL marks coalesced_events_attributes_under_load as an
optional test. Existing asserts for events outside the coalesced
list are covered in another test:
coalesced_events_attributes.https.html.
Fixed: 339686361
Change-Id: Icf01c5592adb7b6f1b6b0a8ae6c61a75f7b6914b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5806434
Reviewed-by: Kevin Ellis kevers@chromium.org
Commit-Queue: Mustaq Ahmed mustaq@chromium.org
Cr-Commit-Position: refs/heads/main@{#1346740}