Add a test for `for await (async of` (#2982)

test/language/statements/for-await-of/head-lhs-async.js

@@ -0,0 +1,23 @@
+// Copyright (C) 2021 Stuart Cook. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-for-in-and-for-of-statements
+description: >
+  The left-hand-side of a for-await-of loop may be the identifier `async`.
+info: |
+  ForInOfStatement[Yield, Await, Return]:
+    [+Await] for await ( [lookahead ≠ let] LeftHandSideExpression[?Yield, ?Await] of AssignmentExpression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return]
+features: [async-iteration]
+flags: [async]
+---*/
+
+let async;
+
+async function fn() {
+  for await (async of [7]);
+}
+
+fn()
+  .then(() => assert.sameValue(async, 7), $DONE)
+  .then($DONE, $DONE);