Allow to run tests without CouchbaseMock.jar

Fixes binary package builders (DEB/RPM)

Change-Id: I780b01ddd2b2c185136258de7e65798467766a49
Reviewed-on: http://review.couchbase.org/28224
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>