Deflake scrollbar-mousedown-move-mouseup test

This test recently started flaking even outside of virtual test
suites.  The reason for increased flakiness was a change to the
action sequence.  Previously there was a pause before and after the
mouse move, but this got removed with the switch to use a similar
gesture via test-driver.

The problem was isolated to middle button drags. Thus, the test
has been split into a pair of promise tests. Also added a ready
promise to guarantee that the required events have been processed
upstream of the asserts.

Testing locally, 7% flake rate without the fix, 0% with the fix.

Bug: 324441376
Change-Id: Idc8297fbc20df43765c1833cf882c9ca0f6587db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5695508
Commit-Queue: Kevin Ellis <kevers@chromium.org>
Reviewed-by: Mustaq Ahmed <mustaq@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1326212}