Add autotest for session_manager RetrieveActiveSessions()

I've added a new DBus method to session_manager to return a dict of currently
active sessions.  This test exercises it.

CQ-DEPEND=I315ee1cecea7c8af6df9968b569a084381ea9167

BUG=chromium:239830
TEST=run this new test

Change-Id: I08959e69845996c345ef85362847370420697801
Reviewed-on: https://gerrit.chromium.org/gerrit/50895
Tested-by: Chris Masone <cmasone@chromium.org>
Reviewed-by: Nikita Kostylev <nkostylev@chromium.org>
Commit-Queue: Chris Masone <cmasone@chromium.org>
2 files changed