Fix _delete_assembly in test_trigger_post functional test

If assembly is still not deleted after 5 attempts,
we make the test fail instead of assertRaises a TimeoutException.
The assertRaises method was also misused since it should take
at least 3 arguments (2 given).

Change-Id: Id53191e801e1106118d584c7a48af0f59e6c7577
Closes-Bug: #1324135