Integrate Zest-based examples into our test suite

[ptziegler]

With this change, we instantiate and verify the correctness of the Zest examples as part of our JUnit test suite. Special care needs to be taken, in order to make sure that the examples can be used in the test environment, but also still be executed as standalone applications.

This means that those snippets must not create a new Display instance, as one is already created by the test suite and multiple displays are forbidden. The snippet is then instantiated by reflectively calling the main method.

Because the snippets block until the shell is closed by the user, a little trick is needed to allow for the tests to be executed. The actual test is scheduled as an asynchronous task before the snippet is created, which is then executed by the call to "Display.readAndDispatch()".

This contribution is done in preparation for the Zest2.0 migration as a means to increase the test coverage and thus spot any changes that are introduced.