[Test Automation] Remove InteractiveTestPrivate inheritance

Previously, functionality that was layered on by derived test suites
also required overriding functionality in InteractiveTestPrivate. This
created a situation where inheritance could only ever be linear, and
could not function in a mixin-type way.

Now, each chunk of private data is separate, and need only register with
the private test object so that certain things are delegated based on
platform/applicability, such as registering action simulators and
callbacks for element dumps.

This is the same way that element tracker, help bubble factory,
interaction test util, and a bunch of other systems work - you layer on
platform- or environment-specific functionality that goes into a list of
delegates.

See follow-up CL for how we can assemble InteractiveBrowserTest via
composition of InteractiveViewsTest and InteractiveBrowserWindowTest,
rather than via strict inheritance only.

Bug: 449972485
Change-Id: Ie2e0506eb03c4c9e6a4b03dc724874f812f78332
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7017411
Reviewed-by: Christopher Grant <cjgrant@chromium.org>
Commit-Queue: Dana Fried <dfried@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1529734}