Fix leak in single-touch.html

This test could trigger leak detection since not waiting on pointerup
after all of the async tests have completed.

Actions().send() does not wait on the dispatched events to be handled.
Each async test marked itself as done once the expected event was
received. Refactored to be a promise driven test.

2 flakes out of 100 runs without the fix.
0 flakes out of 500 runs with the fix.

Bug: 40945204
Change-Id: I5468e3bc67ecdfa5c09e53a705abdcbc22024cde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6033293
Commit-Queue: Kevin Ellis <[email protected]>
Reviewed-by: Mustaq Ahmed <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1386408}