Update banner_theme_test.dart for M3 (flutter#130884)

This PR updates unit tests from banner_theme_test.dart for M3 migration.

More info in flutter#127064

Just added some `Material`prefixes.

packages/flutter/test/material/banner_theme_test.dart

@@ -66,7 +66,7 @@ void main() {
     ]);
   });
 
-  testWidgets('Passing no MaterialBannerThemeData returns defaults', (WidgetTester tester) async {
+  testWidgets('Material3 - Passing no MaterialBannerThemeData returns defaults', (WidgetTester tester) async {
     const String contentText = 'Content';
     final ThemeData theme = ThemeData(useMaterial3: true);
     late final ThemeData localizedTheme;
@@ -115,7 +115,7 @@ void main() {
     expect(divider.color, theme.colorScheme.outlineVariant);
   });
 
-  testWidgets('Passing no MaterialBannerThemeData returns defaults when presented by ScaffoldMessenger', (WidgetTester tester) async {
+  testWidgets('Material3 - Passing no MaterialBannerThemeData returns defaults when presented by ScaffoldMessenger', (WidgetTester tester) async {
     const String contentText = 'Content';
     const Key tapTarget = Key('tap-target');
     final ThemeData theme = ThemeData(useMaterial3: true);
@@ -453,7 +453,7 @@ void main() {
     // support is deprecated and the APIs are removed, these tests
     // can be deleted.
 
-    testWidgets('Passing no MaterialBannerThemeData returns defaults', (WidgetTester tester) async {
+    testWidgets('Material2 - Passing no MaterialBannerThemeData returns defaults', (WidgetTester tester) async {
       const String contentText = 'Content';
 
       await tester.pumpWidget(MaterialApp(
@@ -499,7 +499,7 @@ void main() {
       expect(divider.color, null);
     });
 
-    testWidgets('Passing no MaterialBannerThemeData returns defaults when presented by ScaffoldMessenger', (WidgetTester tester) async {
+    testWidgets('Material2 - Passing no MaterialBannerThemeData returns defaults when presented by ScaffoldMessenger', (WidgetTester tester) async {
       const String contentText = 'Content';
       const Key tapTarget = Key('tap-target');