Add a test of the semantics of About items

These are XBlocks that are detached (not part of the course content hierarchy),
and are also DIRECT_ONLY_CATEGORIES (meaning they are always autopublished).

https://openedx.atlassian.net/browse/PLAT-858 identifies a bug
where an About item isn't correctly deleted in SplitDraft, and this test
reproduces that case.

common/lib/xmodule/xmodule/modulestore/tests/test_semantics.py

+"""
+Tests of modulestore semantics: How do the interfaces methods of ModuleStore relate to each other?
+"""
+
+from xmodule.modulestore.tests.utils import (
+    PureModulestoreTestCase, MongoModulestoreBuilder,
+    SPLIT_MODULESTORE_SETUP
+)
+from xmodule.modulestore.exceptions import ItemNotFoundError
+from xmodule.modulestore import ModuleStoreEnum
+from xmodule.modulestore.tests.factories import CourseFactory
+
+class AboutItemSemantics(PureModulestoreTestCase):
+    """
+    Verify the behavior of About items (which are intentionally orphaned, DIRECT_ONLY/autopublished)
+    blocks intended to store snippets of course content.
+    """
+
+    __test__ = False
+
+    def setUp(self):
+        super(AboutItemSemantics, self).setUp()
+        self.course = CourseFactory.create(
+            org='test_org',
+            number='999',
+            run='test_run',
+            display_name='My Test Course',
+            modulestore=self.store
+        )
+
+        self.about_usage_key = self.course.id.make_usage_key('about', 'video')
+
+    def assertAboutDoesntExist(self):
+        with self.assertRaises(ItemNotFoundError):
+            self.store.get_item(self.about_usage_key, revision=ModuleStoreEnum.RevisionOption.published_only)
+        with self.assertRaises(ItemNotFoundError):
+            self.store.get_item(self.about_usage_key, revision=ModuleStoreEnum.RevisionOption.draft_only)
+
+        with self.store.branch_setting(ModuleStoreEnum.Branch.draft_preferred):
+            with self.assertRaises(ItemNotFoundError):
+                self.store.get_item(self.about_usage_key)
+
+        with self.store.branch_setting(ModuleStoreEnum.Branch.published_only):
+            with self.assertRaises(ItemNotFoundError):
+                self.store.get_item(self.about_usage_key)
+
+    def assertAboutHasContent(self, content):
+        self.assertEquals(
+            content,
+            self.store.get_item(
+                self.about_usage_key,
+                revision=ModuleStoreEnum.RevisionOption.published_only
+            ).data
+        )
+        self.assertEquals(
+            content,
+            self.store.get_item(
+                self.about_usage_key,
+                revision=ModuleStoreEnum.RevisionOption.draft_only
+            ).data
+        )
+
+        with self.store.branch_setting(ModuleStoreEnum.Branch.draft_preferred):
+            self.assertEquals(
+                content,
+                self.store.get_item(
+                    self.about_usage_key,
+                ).data
+            )
+
+        with self.store.branch_setting(ModuleStoreEnum.Branch.published_only):
+            self.assertEquals(
+                content,
+                self.store.get_item(
+                    self.about_usage_key,
+                ).data
+            )
+
+    def test_create(self):
+        self.assertAboutDoesntExist()
+
+        data_string = '<div>test data</div>'
+
+        with self.store.branch_setting(ModuleStoreEnum.Branch.draft_preferred):
+            about_item = self.store.create_xblock(
+                self.course.runtime,
+                self.course.id,
+                self.about_usage_key.block_type,
+                block_id=self.about_usage_key.block_id
+            )
+
+            about_item.data = data_string
+            self.store.update_item(about_item, ModuleStoreEnum.UserID.test, allow_not_found=True)
+
+        self.assertAboutHasContent(data_string)
+
+    def test_update(self):
+        self.test_create()
+
+        with self.store.branch_setting(ModuleStoreEnum.Branch.draft_preferred):
+            about_item = self.store.get_item(self.about_usage_key)
+
+            data_string = "<div>different test data</div>"
+            self.assertNotEquals(data_string, about_item.data)
+
+            about_item.data = data_string
+
+            self.store.update_item(about_item, ModuleStoreEnum.UserID.test, allow_not_found=True)
+
+        self.assertAboutHasContent(data_string)
+
+    def test_delete(self):
+        self.test_create()
+
+        with self.store.branch_setting(ModuleStoreEnum.Branch.draft_preferred):
+            self.store.delete_item(self.about_usage_key, ModuleStoreEnum.UserID.test)
+
+        self.assertAboutDoesntExist()
+
+class TestSplitAboutItemSemantics(AboutItemSemantics):
+    MODULESTORE = SPLIT_MODULESTORE_SETUP
+    __test__ = True
+
+
+class TestMongoAboutItemSemantics(AboutItemSemantics):
+    MODULESTORE = MongoModulestoreBuilder()
+    __test__ = True