#8 - Ensure customers are loaded in UI

Let's implement our first web integration test to verify the UI layer also works as expected

• add a dependency to io.jmix.ui:jmix-ui-test-assist
• use @UiTest annotation to start a web integration test
• Interact with the regular Jmix UI APIs, like screens to run the system under test
• interact with Screen components, like the Table to check if a particular Customer is displayed
• spin up an ad-hoc test database for UI tests, by using Spring Boot's @AutoConfigureTestDatabase
• setup & cleanup test data programmatically through DataManager