Fix QmitkMultiLabelTreeModelTest
Summary: The test cases were outdated after changing the naming of instances from "Label 1 #x" to "Label 1 [x]" in 81c87e38b615. They should be up to date again now.
Test Plan: Tests should pass (specifically QmitkMultiLabelTreeModelTest)
Reviewers: O1 MITK Reviewer Group I, kislinsk, floca
Reviewed By: O1 MITK Reviewer Group I, floca
Differential Revision: https://phabricator.mitk.org/D815