[AXMode Form Controls] Add dump tree tests for form controls mode

This CL parameterizes the accessibility dump test code to take in
and AXMode.

The parameterization supports the addition of a couple dump tree
tests for the form controls AXMode.

AX-Relnotes: N/A
Change-Id: I2f1bc12034e1f444c4ca6449af10b9304fffceba
Bug: 1411856
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4261637


Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Mark Schillaci <mschillaci@google.com>
Commit-Queue: Amanda Lin Dietz <aldietz@google.com>
Cr-Commit-Position: refs/heads/main@{#1118865}