Call ResetDB as Cleanup routine to close existing database connections

on backend unmount.

builtin/logical/postgresql/backend.go

@@ -35,6 +35,8 @@ func Backend() *framework.Backend {
 		Secrets: []*framework.Secret{
 			secretCreds(&b),
 		},
+
+		Clean: b.ResetDB,
 	}
 
 	return b.Backend