test: Improve DropdownMenuFormField tests (flutter#181369)

Resolves
flutter#181367 (comment)

The test was still valid as `DropdownMenuFormField` includes
`DropdownMenu` and therefore could select the desired MenuItemButton.

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Author: Gustl22

packages/flutter/test/material/dropdown_menu_form_field_test.dart

@@ -24,14 +24,6 @@ void main() {
     menuEntries.add(entry);
   }
 
-  Finder findMenuItem(MenuItem menuItem) {
-    // For each menu item there are two MenuItemButton widgets.
-    // The last one is the real button item in the menu.
-    // The first one is not visible, it is part of _DropdownMenuBody
-    // which is used to compute the dropdown width.
-    return find.widgetWithText(MenuItemButton, menuItem.label).last;
-  }
-
   Finder findMenuItemButton(String label) {
     // For each menu items there are two MenuItemButton widgets.
     // The last one is the real button item in the menu.
@@ -40,6 +32,10 @@ void main() {
     return find.widgetWithText(MenuItemButton, label).last;
   }
 
+  Finder findMenuItem(MenuItem menuItem) {
+    return findMenuItemButton(menuItem.label);
+  }
+
   testWidgets('Creates an underlying DropdownMenu', (WidgetTester tester) async {
     await tester.pumpWidget(
       MaterialApp(
@@ -884,7 +880,7 @@ void main() {
     expect(dropdownMenu.restorationId, restorationId);
   });
 
-  testWidgets('Field state is correcly updated', (WidgetTester tester) async {
+  testWidgets('Field state is correctly updated', (WidgetTester tester) async {
     final fieldKey = GlobalKey<FormFieldState<MenuItem>>();
 
     await tester.pumpWidget(
@@ -1366,7 +1362,7 @@ void main() {
     expect(onSelectedCallCount, 1);
   });
 
-  testWidgets('onSelect is called exactly once when reseted', (WidgetTester tester) async {
+  testWidgets('onSelect is called exactly once when reset', (WidgetTester tester) async {
     var onSelectedCallCount = 0;
     final fieldKey = GlobalKey<FormFieldState<MenuItem>>();
     await tester.pumpWidget(
@@ -1436,7 +1432,7 @@ void main() {
     );
 
     // Open the menu.
-    await tester.tap(find.byType(DropdownMenu<String?>));
+    await tester.tap(find.byType(DropdownMenuFormField<String?>));
     await tester.pump();
 
     // Select the 'None' item.