Add multiprofile tests for session_manager.StartSession()

Multiple calls to StartSession() with different users should be allowed,
while multiple calls with the same user should not.

BUG=chromium:235179
TEST=run these new tests

CQ-DEPEND=I1200c4463d43cd8cf7d49a2f9f0667ac1cccc261
CQ-DEPEND=I5ee9fb18e1f8c1a933b899acc62618323f2df9bf

Change-Id: I912d3ded053a7f0f41c604f5ffef15477d634017
Reviewed-on: https://gerrit.chromium.org/gerrit/50223
Commit-Queue: Chris Masone <cmasone@chromium.org>
Reviewed-by: Chris Masone <cmasone@chromium.org>
Tested-by: Chris Masone <cmasone@chromium.org>
diff --git a/client/cros/constants.py b/client/cros/constants.py
index 43ea49b..85a50f7 100644
--- a/client/cros/constants.py
+++ b/client/cros/constants.py
@@ -61,6 +61,8 @@
 
 FLIMFLAM_TEST_PATH = '/usr/lib/flimflam/test/'
 
+GUEST_USER = '$guest'
+
 KEYGEN = 'keygen'
 
 LOGGED_IN_MAGIC_FILE = '/var/run/state/logged-in'