[material_ui] Remove widgets import in dialog_test.dart, dropdown_menu_test.dart and sliver_app_bar_test.dart (#12002)

Part of flutter/flutter#182636 and flutter/flutter#188395

This PR:
* Removed the cross-import of `widgets/semantics_tester.dart`.
* Removed @Skip tag, all tests in this file has passed. `semantics_tester.dart` has existed in material_ui, so we can directly import `semantics_tester.dart`;
* Moved the file to `test/` folder.

## Pre-Review Checklist

Author: QuncCccccc