Use Qt platform "minimal" on Linux (headless) for QmitkAbstractNodeSelectionWidgetTest

Summary: The test fails regularly on headless Linux as Qt isn't continuously able to hook to the virtual framebuffer provided for the test to use the default xcb Qt plugin. On Linux, the minimal platform is available for Qt applications without the need to actually show the GUI.

Test Plan: I already tested it manually on a build client that failed before.

Reviewers: kalali, floca, O3 MITK Reviewer Group II

Reviewed By: kalali, floca, O3 MITK Reviewer Group II

Maniphest Tasks: T27203

Differential Revision: https://phabricator.mitk.org/D263