engine : Fix sporadic failure of BackwardCompatibilityTaskCreationTest

testConcreateCreateTaskBackwardsComaptibility test
in BackwardCompatibilityTaskCreationTest has sporadic
failures due the changes in AsyncTaskFactory.construct
method.

Change-Id: I8d3b24582ed27384e0e46aad454a3c6fafb5130b
Signed-off-by: Ravi Nori <rnori@redhat.com>