test/objectstore: properly enter store context

In the `store_server` test, pass the store to `enter_context`,
instead of the `stack`; the latter is an interesting form of
recursion, and totally not what we want.

test/mod/test_objectstore.py

@@ -303,7 +303,7 @@ class TestObjectStore(unittest.TestCase):
         with contextlib.ExitStack() as stack:
 
             store = objectstore.ObjectStore(self.store)
-            stack.enter_context(stack)
+            stack.enter_context(store)
 
             tmpdir = tempfile.TemporaryDirectory()
             tmpdir = stack.enter_context(tmpdir)