Disable some tests for Julia > v1.6

[simonschoelly]

This PR temporarily disables some tests on Julia > 1.6. We should aim to add them back as soon as possible. There are two different issues:

• The visual regression tests fail. One reason is that we have to explicitly import Gtk.jl. But this is not enough - there is a second error where Gtk wants to open a display on the action runner but fails to do so.
• A second issue is that some of the layouts rely on random generators. As the default random generator is not guaranteed to be stable between Julia versions we generate a different layout on higher Julia versions. We should fix that by using StableRNGs.jl.