Fix CMake variable type of MITK_XVFB_TESTING_COMMAND

Summary: CMake variable type was wrong passing the value to the MITK build

Test Plan: CI

Reviewers: O1 MITK Reviewer Group I, kislinsk

Reviewed By: O1 MITK Reviewer Group I, kislinsk

Differential Revision: https://phabricator.mitk.org/D327