fix: support parsing the pointer-tag OLX for external XBlocks (#37133)

Previously, the built-in XBlocks (problem, video, etc) could be parsed
from pointer tag syntax, but externally-defined XBlocks (drag-and-drop,
ORA, etc.) could only be parsed from inline syntax. This is because the
built-in blocks have special parsing logic, defined in XmlMixin,
which is not available to external blocks.

This PR shifts the pointer tag parsing "up a level" such that the parent
blocks parse the pointer tag, regardless of whether the child is built-in
or external:
* vertical (aka unit)
* split_test (aka content experiment)
* itembank (aka problem bank)
* library_content (aka randomized legacy library)

The following parent blocks still lack support for external pointer-tag children;
we will fix this in a follow-up PR:
* randomize
* all externally defined container blocks

Part of: https://github.com/openedx/XBlock/issues/823

xmodule/tests/xml/__init__.py

@@ -42,13 +42,14 @@ class InMemorySystem(XMLParsingSystem, MakoDescriptorSystem):  # pylint: disable
         )
         self.id_generator = Mock()
 
-    def process_xml(self, xml):  # pylint: disable=method-hidden
+    def process_xml(self, xml, def_id=None, def_loaded=False):  # pylint: disable=method-hidden
         """Parse `xml` as an XBlock, and add it to `self._blocks`"""
         self.get_asides = Mock(return_value=[])
         block = self.xblock_from_node(
             etree.fromstring(xml),
             None,
             CourseLocationManager(self.course_id),
+            def_id,
         )
         self._blocks[str(block.location)] = block
         return block
@@ -61,7 +62,7 @@ class InMemorySystem(XMLParsingSystem, MakoDescriptorSystem):  # pylint: disable
 class XModuleXmlImportTest(TestCase):
     """Base class for tests that use basic XML parsing"""
     @classmethod
-    def process_xml(cls, xml_import_data):
+    def process_xml(cls, xml_import_data, def_id=None, def_loaded=False):
         """Use the `xml_import_data` to import an :class:`XBlock` from XML."""
         system = InMemorySystem(xml_import_data)
-        return system.process_xml(xml_import_data.xml_string)
+        return system.process_xml(xml_import_data.xml_string, def_id)