Rename layout_utils.dart to rendering_tester.dart

Also, rename build_utils.dart to widget_tester.dart. These files are now named
for their most commonly used classes.

Finally, add a .analysis_options to silence the (intentional) analyzer warnings
in append_child_test.dart.

packages/unit/.analysis_options

+analyzer:
+  exclude:
+    - 'test/engine/append_child_test.dart'

packages/unit/test/examples/sector_layout_test.dart

 import 'package:test/test.dart';
-import '../rendering/layout_utils.dart';
+
+import '../rendering/rendering_tester.dart';
 import '../../../../examples/rendering/sector_layout.dart';
 
 void main() {

packages/unit/test/rendering/box_test.dart

@@ -3,7 +3,7 @@ import 'package:sky/theme/colors.dart' as colors;
 import 'package:sky/theme/shadows.dart';
 import 'package:test/test.dart';
 
-import 'layout_utils.dart';
+import 'rendering_tester.dart';
 
 void main() {
   test("should size to render view", () {

packages/unit/test/rendering/image_test.dart

@@ -3,7 +3,7 @@ import 'dart:sky' as sky;
 import 'package:sky/rendering.dart';
 import 'package:test/test.dart';
 
-import 'layout_utils.dart';
+import 'rendering_tester.dart';
 
 class SquareImage implements sky.Image {
   int get width => 10;

packages/unit/test/rendering/shrink_wrap_test.dart

 import 'package:sky/rendering.dart';
 import 'package:test/test.dart';
 
-import 'layout_utils.dart';
+import 'rendering_tester.dart';
 
 class RenderTestBox extends RenderBox {
   RenderTestBox(this._intrinsicDimensions);

packages/unit/test/rendering/stack_test.dart

 import 'package:sky/rendering.dart';
 import 'package:test/test.dart';
 
-import 'layout_utils.dart';
+import 'rendering_tester.dart';
 
 void main() {
   test('Stack can layout with top, right, bottom, left 0.0', () {

packages/unit/test/rendering/viewport_test.dart

 import 'package:sky/rendering.dart';
 import 'package:test/test.dart';
 
-import 'layout_utils.dart';
+import 'rendering_tester.dart';
 
 void main() {
   test('Should be able to hit with negative scroll offset', () {

packages/unit/test/widget/align_test.dart

 import 'package:sky/widgets.dart';
 import 'package:test/test.dart';
 
-import 'build_utils.dart';
+import 'widget_tester.dart';
 
 void main() {
   test('Align smoke test', () {

packages/unit/test/widget/box_decoration_test.dart

@@ -2,7 +2,7 @@ import 'package:sky/theme/colors.dart' as colors;
 import 'package:sky/widgets.dart';
 import 'package:test/test.dart';
 
-import 'build_utils.dart';
+import 'widget_tester.dart';
 
 void main() {
   test('Circles can have uniform borders', () {

packages/unit/test/widget/date_picker_test.dart

 import 'package:sky/widgets.dart';
 import 'package:test/test.dart';
 
-import 'build_utils.dart';
+import 'widget_tester.dart';
 
 void main() {
   test('Can select a day', () {

packages/unit/test/widget/global_key_test.dart

 import 'package:sky/widgets.dart';
 import 'package:test/test.dart';
 
-import 'build_utils.dart';
+import 'widget_tester.dart';
 
 void main() {
   test('Global keys notify add and remove', () {

packages/unit/test/widget/mimic_test.dart

 import 'package:sky/widgets.dart';
 import 'package:test/test.dart';
 
-import 'build_utils.dart';
+import 'widget_tester.dart';
 
 void main() {
   test('Mimic can track tree movements', () {

packages/unit/test/widget/progress_indicator_test.dart

@@ -3,7 +3,7 @@ import 'package:sky/rendering.dart';
 import 'package:sky/widgets.dart';
 import 'package:test/test.dart';
 
-import 'build_utils.dart';
+import 'widget_tester.dart';
 
 void main() {
   test('LinearProgressIndicator changes when its value changes', () {