[Desktop Android CRX] Compile sources in //extensions/test

We are experimenting with desktop-android configurations for Chrome.

This CL adds the //extensions:test_support target to the experimental
desktop-android build config. This includes lots of test utilities
in //extensions/test, as well as a few test utilities in other
directories. A few of these sources are still not compiled due to
dependencies they have on other pieces (platform apps, guest view, or
other APIs).

This is currently experimental and has no production behavior changes.

Cq-Include-Trybots: luci.chromium.try:android-desktop-arm64-compile-rel,android-desktop-x64-compile-rel

Bug: 356905053
Change-Id: Id3c3b73df13cbc14b55de56ca8e31126e9fcc619
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5789202
Reviewed-by: David Bertoni <dbertoni@chromium.org>
Commit-Queue: Devlin Cronin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1342027}