Fix session expiration test.

The session expiration test would occasionally fail because two sessions ended up expiring
at approximately the same time. This patch ensures that no session is active when the
initial metric value is polled.

Change-Id: Ib62e7c23fb0c43e0e8ee0c17770d47df19964117
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4502
Reviewed-by: Ishaan Joshi <ishaan@cloudera.com>
Tested-by: jenkins