Split out AppBar/SliverAppBar material tests (#142560)

While looking into resolving https://github.com/flutter/flutter/issues/117903, I found the massive test file `app_bar_test.dart` and found it unwieldy to work with. So before proposing a solution to #117903, which would touch many of these tests, I figured a clean up would be best first.

This splits up `app_bar_test.dart` with a new file `app_bar_sliver_test.dart`, and adds `app_bar_utils.dart` for shared test methods.

It basically moves all SliverAppBar tests into their own file, leaving just AppBar tests in the original file.

packages/flutter/test/material/app_bar_utils.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+Finder findAppBarMaterial() {
+  return find.descendant(
+    of: find.byType(AppBar),
+    matching: find.byType(Material),
+  ).first;
+}
+
+Color? getAppBarBackgroundColor(WidgetTester tester) {
+  return tester.widget<Material>(findAppBarMaterial()).color;
+}
+
+double appBarHeight(WidgetTester tester) {
+  return tester.getSize(find.byType(AppBar, skipOffstage: false)).height;
+}
+
+double appBarTop(WidgetTester tester) {
+  return tester.getTopLeft(find.byType(AppBar, skipOffstage: false)).dy;
+}
+
+double appBarBottom(WidgetTester tester) {
+  return tester.getBottomLeft(find.byType(AppBar, skipOffstage: false)).dy;
+}
+
+double tabBarHeight(WidgetTester tester) {
+  return tester.getSize(find.byType(TabBar, skipOffstage: false)).height;
+}
+
+ScrollController primaryScrollController(WidgetTester tester) {
+  return PrimaryScrollController.of(
+    tester.element(find.byType(CustomScrollView))
+  );
+}
+
+void verifyTextNotClipped(Finder textFinder, WidgetTester tester) {
+  final Rect clipRect = tester.getRect(
+    find.ancestor(of: textFinder, matching: find.byType(ClipRect)).first,
+  );
+  final Rect textRect = tester.getRect(textFinder);
+  expect(textRect.top, inInclusiveRange(clipRect.top, clipRect.bottom));
+  expect(textRect.bottom, inInclusiveRange(clipRect.top, clipRect.bottom));
+  expect(textRect.left, inInclusiveRange(clipRect.left, clipRect.right));
+  expect(textRect.right, inInclusiveRange(clipRect.left, clipRect.right));
+}