autocomplete test [nfc]: Fix brittle avatar image finder

Add `findAvatarImage` to check userId instead of URL,
making it resilient to changes in avatar image details like size.
Previous finder used 'findNetworkImage' which would fail if the avatar size is different

Relevant Discussion:
https://chat.zulip.org/#narrow/channel/516-mobile-dev-help/topic/ComposeAutocomplete.20test.20failure.20help
#995 (comment)

Author: fombalang

test/widgets/autocomplete_test.dart

@@ -13,6 +13,7 @@ import 'package:zulip/model/localizations.dart';
 import 'package:zulip/model/narrow.dart';
 import 'package:zulip/model/store.dart';
 import 'package:zulip/model/typing_status.dart';
+import 'package:zulip/widgets/content.dart';
 import 'package:zulip/widgets/message_list.dart';
 
 import '../api/fake_api.dart';
@@ -126,10 +127,13 @@ void main() {
   TestZulipBinding.ensureInitialized();
 
   group('@-mentions', () {
+
+    Finder findAvatarImage(int userId) =>
+        find.byWidgetPredicate((widget) => widget is AvatarImage && widget.userId == userId);
+
     void checkUserShown(User user, PerAccountStore store, {required bool expected}) {
       check(find.text(user.fullName).evaluate().length).equals(expected ? 1 : 0);
-      final avatarFinder =
-        findNetworkImage(store.tryResolveUrl(user.avatarUrl!).toString());
+      final avatarFinder = findAvatarImage(user.userId);
       check(avatarFinder.evaluate().length).equals(expected ? 1 : 0);
     }