EXO: Use base::TestSuite on non-CrOS builds

Allows running tests without ChromeOS dependencies (Ash).
Changes:
 * Add ExoTestSuiteAura to initialize state that would otherwise be
 initialized by the AshTestSuite.
 * Initialize the test InputMethodFactory otherwise tests using input
 will DCHECK() when using ExoTestBaseAura.

Bug: 896710
Test: Run exo_unittests from a Chromecast build
Change-Id: I709bae42574442cd3d46e834b19f97f8a20a5505
Reviewed-on: https://chromium-review.googlesource.com/c/1345242
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615209}
5 files changed