Disable SystemWebAppAccessibilityTest for ChromeVox in manifest v3

This test is failing when ChromeVox runs in manifest v3 due to timing
issues. In general, manifest v3 extensions take longer to initialize
which is exposing timing issues that were covered up in manifest v2.
Specifically, the keyboard shortcut to jump to the next pane fails
because the accelerator isn't bound at the time of keypress, causing
the test to timeout.

I've disabled the test with a TODO to unblock
https://crrev.com/c/6962374, which adds a finch field trial for
ChromeVox in manifest v3.

Note that the scenario in the test works correctly when I test
manually, so it's just a matter of fixing the timing issue so that
the test can pass.

Bug: 433771715
Change-Id: I5651973248460d65d58516c92a30e1c90ef7df52
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7016514
Reviewed-by: Jimmy Gong <jimmyxgong@chromium.org>
Commit-Queue: Akihiro Ota <akihiroota@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1528262}