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This change adds a base class for panel tests and makes
ChromeVoxPanelTest and ChromeVoxTutorialTest inherit from it.

Bug: 1127967
Change-Id: I68131a5d12e79910779cd41ad0ae51a923d6f214
AX-Relnotes: N/A
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2568685
Commit-Queue: Akihiro Ota <akihiroota@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833893}