TBR tests: properly clean up tmp dir

Change-Id: I8fdd3e2b77ac1d9ab113eb3560b4d3765e2e5e9e
diff --git a/x/ref/services/syncbase/server/watchable/test_util.go b/x/ref/services/syncbase/server/watchable/test_util.go
index 77aab33..cfb254c 100644
--- a/x/ref/services/syncbase/server/watchable/test_util.go
+++ b/x/ref/services/syncbase/server/watchable/test_util.go
@@ -36,9 +36,10 @@
 			st.Close()
 		}
 	} else {
-		st = createLevelDB(getPath())
+		path := getPath()
+		st = createLevelDB(path)
 		return st, func() {
-			destroyLevelDB(st, getPath())
+			destroyLevelDB(st, path)
 		}
 	}
 }