Add a Domain to WorkflowExecution objects

This will be needed later for finding an activity type for instance.

tests/test_swf/utils.py

@@ -1,8 +1,14 @@
 from moto.swf.models import (
+    Domain,
     WorkflowType,
 )
 
 
+# A test Domain
+def get_basic_domain():
+    return Domain("test-domain", "90")
+
+
 # A generic test WorkflowType
 def _generic_workflow_type_attributes():
     return [