Move db related unittests to proper places

It is expected, that unittests cover project modules in relation 1:1 and
placed in appropriate tree as main modules do.

It should be like in example:
manila/[tests/]foo/bar/[test_]modulename.py

Change-Id: I890deaeb41b87a6ae82868905144fd7477272058
Partial-Bug: #1348633

manila/tests/db/test_migration.py

@@ -20,9 +20,9 @@ from manila.db import migration
 from manila import test
 
 
-class AlembicCommandTestCase(test.TestCase):
+class MigrationTestCase(test.TestCase):
     def setUp(self):
-        super(AlembicCommandTestCase, self).setUp()
+        super(MigrationTestCase, self).setUp()
         self.config_patcher = mock.patch(
             'manila.db.migrations.alembic.migration._alembic_config')
         self.config = self.config_patcher.start()