[bedrock] Eliminate bespoke Browser management from extensions tests

TestWithBrowserView is deprecated and actively being eliminated.

To help simplify the transition this CL centralizes the creation of
additional Browser-with-BrowserView instances in the
TestWithBrowserView fixture.

Both ExtensionsMenuViewUnitTest and ExtensionsToolbarContainerUnitTest
have been updated to leverage this new API.

Bug: 413168662
Change-Id: I9e0094cc139f4d0e8bbe3d9679fc1132229ab362
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6725974
Commit-Queue: Tom Lukaszewicz <tluk@chromium.org>
Reviewed-by: Keren Zhu <kerenzhu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1485396}