msglist-test: Refactor some logic to use prepareBoringImageHttpClient

In the test ('Updates avatar on RealmUserUpdateEvent'),
the code for setting a `FakeImageHttpClient` is
replaced by the `prepareBoringImageHttpClient` function.

Author: sm-sayedi

test/widgets/message_list_test.dart

@@ -467,11 +467,7 @@ void main() {
         await tester.pump();
       }
 
-      final httpClient = FakeImageHttpClient();
-      debugNetworkImageHttpClientProvider = () => httpClient;
-      httpClient.request.response
-        ..statusCode = HttpStatus.ok
-        ..content = kSolidBlueAvatar;
+      prepareBoringImageHttpClient();
 
       await setupMessageListPage(tester, messageCount: 10);
       checkResultForSender(eg.selfUser.avatarUrl);