checkNoVerificationOnServerRestartWith2Clients(): Close self._storage

too.  Mystery:  whether this test dies with permission denied when
trying to unlink the .zec file is still a crapshoot.   Don't know why.
At least four other tests show sporadic failures of the same kind.

src/ZEO/tests/ConnectionTests.py

@@ -728,6 +728,7 @@ class ReconnectionTests(CommonSetupTearDown):
         self.assertEqual(self._storage.verify_result, "no verification")
         self.assertEqual(perstorage.verify_result, "no verification")
         perstorage.close()
+        self._storage.close()
 
     def checkQuickVerificationWith2Clients(self):
         perstorage = self.openClientStorage(cache="test")