content test [nfc]: Have prepareContentBare prepare test image HTTP client

Author: chrisbobbe

test/widgets/content_test.dart

@@ -117,6 +117,8 @@ void main() {
     widget = GlobalStoreWidget(child: widget);
     addTearDown(testBinding.reset);
 
+    prepareBoringImageHttpClient();
+
     final pushedRoutes = <Route<dynamic>>[];
     final testNavObserver = TestNavigatorObserver()
       ..onPushed = (route, prevRoute) => pushedRoutes.add(route);
@@ -134,6 +136,8 @@ void main() {
       await tester.pump();
     }
 
+    debugNetworkImageHttpClientProvider = null;
+
     // `tester.pumpWidget` introduces an initial route;
     // remove it so consumers only have newly pushed routes.
     assert(pushedRoutes.length == 1);