test: Update a BlockStructureFactory test mixin.

Previously, we were not caching BlockStructures to the database when we
were adding them to the store by default.  Now that we are doing that,
the BlockStructureFactory test failed because it was taking a shortcut
that would no longer work.  It was just creating a blockstructure that
had relations but not any block information.

Now that we're always persisting updates to the database, this broke
because to persist the structure to the database, we have to look up the
block information from the block_structure which now fails.

This change updates the test mixin to add more data so that the content
can be persisted to the database successfully as a part of this test.

openedx/core/djangoapps/content/block_structure/tests/helpers.py

@@ -7,6 +7,7 @@
 from uuid import uuid4
 from unittest.mock import patch
 
+from opaque_keys.edx.keys import CourseKey
 from opaque_keys.edx.locator import BlockUsageLocator, CourseLocator
 
 from xmodule.modulestore.exceptions import ItemNotFoundError
@@ -264,7 +265,7 @@ def block_key_factory(self, block_id):
         Override this method if the block_key should be anything
         different from the index integer values in the Children Maps.
         """
-        return block_id
+        return CourseKey.from_string("course-v1:org+course+run").make_usage_key("html", str(block_id))
 
     def create_block_structure(self, children_map, block_structure_cls=BlockStructureBlockData):
         """
@@ -274,10 +275,12 @@ def create_block_structure(self, children_map, block_structure_cls=BlockStructur
         # create empty block structure
         block_structure = block_structure_cls(root_block_usage_key=self.block_key_factory(0))
 
-        # _add_relation
+        # _add_relation and blocks
         for parent, children in enumerate(children_map):
+            block_structure._get_or_create_block(self.block_key_factory(parent))
             for child in children:
                 block_structure._add_relation(self.block_key_factory(parent), self.block_key_factory(child))  # pylint: disable=protected-access
+                block_structure._get_or_create_block(self.block_key_factory(child))
         return block_structure
 
     def get_parents_map(self, children_map):