reenable UTF-8 BOM tests (rustwasm#2031)

Author: alexcrichton

tests/headless/strings.js

@@ -13,8 +13,7 @@ export function test_string_roundtrip(f) {
   test('a longer string');
   test('a longer 💖 string');
 
-  // TODO re-enable this when Firefox 70 is released
-  //test('\uFEFFbar');
+  test('\uFEFFbar');
 }
 
 export function identity(s) {

tests/headless/strings.rs

@@ -12,6 +12,5 @@ extern "C" {
 fn string_roundtrip() {
     test_string_roundtrip(&Closure::wrap(Box::new(|s| s)));
 
-    // TODO re-enable this when Firefox 70 is released
-    //assert_eq!("\u{feff}bar", &identity("\u{feff}bar"));
+    assert_eq!("\u{feff}bar", &identity("\u{feff}bar"));
 }