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The current pattern for test suites that *may* use aura has the test
suite create aura::Env. In order to enable mus more broadly we need to
make it so that the test suite configures Env appropriate for mus.

This patch adds AuraTestSuiteSetup that is meant to setup the
necessary state at the test suite level for tests that may use
aura. It looks at the command line and potentially enables mus as
appropriate.

In order to setup env for mus WindowTreeClient needs to be configured,
which required moving around some stubs. I had to update
AuraTestHelper for mus being configured at the global level. This
is mildly complicated by the fact that AuraTestHelper wants to create
its own WindowTreeClient in this case.

BUG=776514
TEST=covered by tests

Change-Id: Ie433e4f6afe58b2aa15bdfb08fe462c6c1a30fad
Reviewed-on: https://chromium-review.googlesource.com/792455
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Elliot Glaysher <erg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519884}