[SxS] Add interactive ui test mixin for split tabs

As we write more of these tests, it'll be helpful to have a common place
for all shared split-tab related actions.

Change-Id: Idf3d50ad902501607e0190eaf450efd08da0df60
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6463814
Commit-Queue: Alison Gale <agale@chromium.org>
Reviewed-by: Eshwar Stalin <estalin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1448355}