Add a test that verifies an old bug with join
Add a test that verifies an old bug with join

* There was a bug in stable/libery that occurred in case when we
  had a "join" task that joined multiple branches sourced from
  the same task. In this case line 302 (in the current revision)
  in direct_workflow.py caused IndexError when evaluating in_t_ex[-1]
  as described in the bug. This bug was implicitly fixed since then
  but a test for it was missing. This patch adds a test to prevent
  this bug occurrence in the future.

Change-Id: If277989cc9149b219a9c63287290daa321a3e438
Closes-Bug: #1539173

openstack/mistral,openstack/mistral,StackStorm/mistral,StackStorm/mistral