Migrate the following tests to JUnit 5:
- StackRendererTest
- ToolBarManagerRendererTest
- MPartTest
- MWindowTest
- PartOnTopManagerTest
- MenuManagerRendererTest
- PartRenderingEngineTests

This involves:
- Replacing WorkbenchContextRule with WorkbenchContextExtension
- Updating imports to org.junit.jupiter.api.*
- Updating annotations (@Before -> @BeforeEach, etc.)
- Swapping assertion arguments where necessary
- Updating assumptions
- Adding org.opentest4j dependency for assumptions