assert if mock file remove fails. Use env for test dir

Change-Id: I88640746259cd7d954d07e7d760deb1dc504272b
Reviewed-on: http://review.couchbase.org/39981
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>