Harden spanner system tests further. (#4374)

Because of quota, we are sharing the instance we use to test CRUD of
databases: we therefore cannot rely on having only the expected databases
present, as other tests may be running simultaneously, or may have failed
in ways which left 'stray' databases in the instance.

spanner/tests/system/test_system.py

@@ -274,12 +274,10 @@ def test_create_database(self):
         # We want to make sure the operation completes.
         operation.result(30)  # raises on failure / timeout.
 
-        name_attr = operator.attrgetter('name')
-        expected = sorted([temp_db, self._db], key=name_attr)
-
-        databases = list(Config.INSTANCE.list_databases())
-        found = sorted(databases, key=name_attr)
-        self.assertEqual(found, expected)
+        database_ids = [
+            database.database_id
+            for database in Config.INSTANCE.list_databases()]
+        self.assertIn(temp_db_id, database_ids)
 
     def test_update_database_ddl(self):
         pool = BurstyPool()