[test] Spline tests causes gnuplot timeout

When running the test_cubicspline on a headless machine (e.g., hal), I get the following message and the tests runs for more than 60 s due to a timeout

qt.qpa.xcb: could not connect to display 
qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, xcb.

Could not connect to existing gnuplot_qt. Starting a new one.
qt.qpa.xcb: could not connect to display 
qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, xcb.

Not sure if some packages are missing on hal or if we need to you the gnuplot interface differently, e.g., by setting

gnuplot.setOpenPlotWindow(false);