chromeos: adds ability for AshTestHelper to create MusClient

This is useful for tests targetting single-process-mash. This also
exposes a function on AshTestBase to create MusClient, which is useful
for tests outside of ash that still use AshTestBase. The later is needed
to ensure unit_tests work with SingleProcessMash.

BUG=910241
TEST=test only changes

Change-Id: I97d095b55eba1ba0f19fa59358a999436fd9dc73
Reviewed-on: https://chromium-review.googlesource.com/c/1375454
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616369}
8 files changed