test: fix the illegal syntax in a module for test

this is related to #1792, after removing the ilegal code,
the code still cover the cases except case 3.

test/cases/parsing/harmony-export-precedence/a.js

@@ -1,5 +1,5 @@
 export function a() { return "a1"; }
-export { a, b } from "./b";
+export { b } from "./b";
 export * from "./c";
-export { d, e } from "./b";
+export { d } from "./b";
 export var e = "e1";