aura: Create a single Env instance for aura_unittests.
Create the Env instance for running the tests in AuraTestSuite. Also fix some
flaky tests that used to install pre-target handlers on the root-window, but
did not remove them. This can cause a crash since root_window() outlives the
pre-target handlers, and synthetic mouse-events can be dispatched to the
root-window during tear-down.
BUG=none
R=sky@chromium.org
Review URL: https://codereview.chromium.org/
1287073004 .
Cr-Commit-Position: refs/heads/master@{#343760}