Make test for vaadin-spring-starter

[denis-anisimov]

This ticket may be should be in the platform.

The test should check that vaadin-spring-starter transitively introduces dependency to vaadin-core

Fix for https://github.com/vaadin/spring/issues/356 removes vaadin-spring-starter dependency from our Spring add-on tests.

vaadin-spring-starter is a part of platform and it's build separately and can't be a part of our regular tests for Spring add-on. There should be either a separate testing module for it or it should be tested in the platform and not in the Sprig repo at all.

[denis-anisimov]

The original test wasn't properly done since it tested only Button dependency . But the test should test vaadin-core dependency.