Make CcxFactory fill in a coach if one isn't supplied

lms/djangoapps/ccx/tests/factories.py

 """
 Dummy factories for tests
 """
+from factory import SubFactory
 from factory.django import DjangoModelFactory
+from student.tests.factories import UserFactory
 from ccx.models import CustomCourseForEdX  # pylint: disable=import-error
 from ccx.models import CcxMembership  # pylint: disable=import-error
 from ccx.models import CcxFutureMembership  # pylint: disable=import-error
@@ -11,6 +13,7 @@ class CcxFactory(DjangoModelFactory):  # pylint: disable=missing-docstring
     FACTORY_FOR = CustomCourseForEdX
     display_name = "Test CCX"
     id = None  # pylint: disable=redefined-builtin, invalid-name
+    coach = SubFactory(UserFactory)
 
 
 class CcxMembershipFactory(DjangoModelFactory):  # pylint: disable=missing-docstring