Add in-memory storage.

Test Plan

Land the two parent diffs, then rebase this one on top of them.

---

Migrated from D645 (view on Phabricator)

swh/storage/tests/test_in_memory.py

+# Copyright (C) 2018  The Software Heritage developers
+# See the AUTHORS file at the top-level directory of this distribution
+# License: GNU General Public License version 3, or any later version
+# See top-level LICENSE file for more information
+import unittest
+
+import pytest
+
+from swh.storage.in_memory import Storage
+
+from swh.storage.tests.test_storage import CommonTestStorage
+
+
+class TestInMemoryStorage(CommonTestStorage, unittest.TestCase):
+    """Test the in-memory storage API
+
+    This class doesn't define any tests as we want identical
+    functionality between local and remote storage. All the tests are
+    therefore defined in CommonTestStorage.
+    """
+    def setUp(self):
+        super().setUp()
+        self.storage = Storage()
+
+    @pytest.mark.skip('postgresql-specific test')
+    def test_content_add(self):
+        pass
+
+    @pytest.mark.skip('postgresql-specific test')
+    def test_skipped_content_add(self):
+        pass