Remove reference to MaterialApp and showCupertinoModalPopup from CupertinoAlertDialog (#150725)

This PR removes the usage of Material widgets from unit tests of `CupertinoAlertDialog`. Other than it being just wrong, it also introduces bad behavior, such as the scroll view can't be overscrolled so that the overscroll behavior can't be tested.
* Since there are no longer M2 or M3 variants of tests, I straight up rewrote the unit tests for "default look" with similar tests as those of `CupertinoActionSheet` ([here](https://github.com/flutter/flutter/blob/master/packages/flutter/test/cupertino/action_sheet_test.dart#L21))

This PR also replaces `showCupertinoModalPopup` with `showCupertinoDialog` in `CupertinoAlertDialog`'s example code. The former should only be used by `CupertinoActionSheet`, which has a different animation from the correct `showCupertinoDialog`.

examples/api/lib/cupertino/dialog/cupertino_alert_dialog.0.dart

@@ -23,9 +23,8 @@ class AlertDialogApp extends StatelessWidget {
 class AlertDialogExample extends StatelessWidget {
   const AlertDialogExample({super.key});
 
-  // This shows a CupertinoModalPopup which hosts a CupertinoAlertDialog.
   void _showAlertDialog(BuildContext context) {
-    showCupertinoModalPopup<void>(
+    showCupertinoDialog<void>(
       context: context,
       builder: (BuildContext context) => CupertinoAlertDialog(
         title: const Text('Alert'),