autocomplete test: Fix not-starve test, using awaitFakeAsync

Using just `fakeAsync`, when this hit an `await` it just stopped and
didn't finish the remainder of the test, so didn't get to the point
of testing what it's meant to test.

I believe the test worked correctly when first committed, as it had no
`await` of its own; but later was accidentally defeated by eca33f9
introducing an `await` for `store.addUsers`.

Using our `awaitFakeAsync` fixes the problem.

This was the only call to `fakeAsync` in our codebase, so I believe
this commit fixes the whole problem.

Author: gnprice

test/model/autocomplete_test.dart

@@ -2,7 +2,6 @@ import 'dart:async';
 import 'dart:convert';
 
 import 'package:checks/checks.dart';
-import 'package:fake_async/fake_async.dart';
 import 'package:flutter/widgets.dart';
 import 'package:test/scaffolding.dart';
 import 'package:zulip/api/model/initial_snapshot.dart';
@@ -15,6 +14,7 @@ import 'package:zulip/widgets/compose_box.dart';
 
 import '../api/fake_api.dart';
 import '../example_data.dart' as eg;
+import '../fake_async.dart';
 import 'test_store.dart';
 import 'autocomplete_checks.dart';
 
@@ -189,7 +189,7 @@ void main() {
   });
 
   test('MentionAutocompleteView not starve timers', () {
-    fakeAsync((binding) async {
+    return awaitFakeAsync((binding) async {
       const narrow = ChannelNarrow(1);
       final store = eg.store();
       await store.addUsers([eg.selfUser, eg.otherUser, eg.thirdUser]);