[wasm] fix testharness - unreached_func

See usage in the js-api tests; previously it would have thrown without
executing any tests. Now, it can be used to generate trapping functions.

Bug: v8:8319
Change-Id: Ia1643d8f337a10ea86c1e700c7702ed7d3ed0c97
Reviewed-on: https://chromium-review.googlesource.com/c/1352298Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57885}

test/wasm-js/testharness.js

@@ -19,7 +19,7 @@ let lastPromise = Promise.resolve();
 
 function test(func, description) {
   let maybeErr;
-  try { func({unreached_func: assert_unreached}); }
+  try { func({unreached_func: unreached_func}); }
   catch(e) { maybeErr = e; }
   if (typeof maybeErr !== 'undefined') {
     console.log(`${description}: FAIL. ${maybeErr}`);
@@ -98,6 +98,12 @@ function assert_array_equals(actual, expected, description) {
   }
 }
 
+function unreached_func(msg) {
+  return function trap() {
+    assert_unreached(msg);
+  };
+}
+
 function assert_unreached(description) {
   throw new Error(`unreachable:\n${description}`);
 }