fix unstable Test

MainMenuActivityTest class should be now stable.

https://jira.catrob.at/browse/CAT-1048

Testrun: https://jenkins.catrob.at/view/All-Categories/view/Catroid-multi-job/job/Catroid-Multi-Job-Custom-Branch/1696/
wouldn't it be better to use something like waitForText(R.string.new_project_dialog_title) or similiar to really avoid future timing problems? instead of solo.sleeps