test [nfc]: Move prepareBoringImageHttpClient to test_images.dart

This was already being used for tests outside the test file it was
defined in, and we'd like to start using it in an additional
different test file. So, define it centrally in this place that
makes sense.

Author: chrisbobbe

test/test_images.dart

@@ -1,8 +1,26 @@
 import 'dart:async';
 import 'dart:io';
 
+import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
+/// Set [debugNetworkImageHttpClientProvider] to return a constant image.
+///
+/// Returns the [FakeImageHttpClient] that handles the requests.
+///
+/// The caller must set [debugNetworkImageHttpClientProvider] back to null
+/// before the end of the test.
+// TODO(upstream) simplify callers by using addTearDown: https://github.com/flutter/flutter/issues/123189
+//   See also: https://github.com/flutter/flutter/issues/121917
+FakeImageHttpClient prepareBoringImageHttpClient() {
+  final httpClient = FakeImageHttpClient();
+  debugNetworkImageHttpClientProvider = () => httpClient;
+  httpClient.request.response
+    ..statusCode = HttpStatus.ok
+    ..content = kSolidBlueAvatar;
+  return httpClient;
+}
+
 class FakeImageHttpClient extends Fake implements HttpClient {
   final FakeImageHttpClientRequest request = FakeImageHttpClientRequest();
 

test/widgets/autocomplete_test.dart

@@ -14,7 +14,7 @@ import '../api/fake_api.dart';
 import '../example_data.dart' as eg;
 import '../model/binding.dart';
 import '../model/test_store.dart';
-import 'content_test.dart';
+import '../test_images.dart';
 
 /// Simulates loading a [MessageListPage] and tapping to focus the compose input.
 ///

test/widgets/content_test.dart

@@ -1,5 +1,3 @@
-import 'dart:io';
-
 import 'package:checks/checks.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
@@ -27,23 +25,6 @@ import 'dialog_checks.dart';
 import 'message_list_checks.dart';
 import 'page_checks.dart';
 
-/// Set [debugNetworkImageHttpClientProvider] to return a constant image.
-///
-/// Returns the [FakeImageHttpClient] that handles the requests.
-///
-/// The caller must set [debugNetworkImageHttpClientProvider] back to null
-/// before the end of the test.
-// TODO(upstream) simplify callers by using addTearDown: https://github.com/flutter/flutter/issues/123189
-//   See also: https://github.com/flutter/flutter/issues/121917
-FakeImageHttpClient prepareBoringImageHttpClient() {
-  final httpClient = FakeImageHttpClient();
-  debugNetworkImageHttpClientProvider = () => httpClient;
-  httpClient.request.response
-    ..statusCode = HttpStatus.ok
-    ..content = kSolidBlueAvatar;
-  return httpClient;
-}
-
 void main() {
   // For testing a new content feature:
   //