Refactor DropdownMenu tests (#157913)

## Description

This PR introduces some utility functions to simplify some DropdownMenu tests.
The main purpose is to centralize and document how tests should find menu items, because it is tricky as there are two occurrences of each widgets and using '.last' is mandatory:

```dart
  Finder findMenuItemButton(String label) {
    // For each menu items 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, label).last;
  }
```

## Related Issue

This is extracted from flutter/flutter#157496.

## Tests

Refactors many existing tests.

Author: bleroux