This way you can assign the test windows to an empty workspace to avoid
interacting with them (when xvfb-run is not an option):
assign [instance="i3test"] workspace testing
Commit 287a0b4 introduced a segfault when validating the i3 config
as the root_screen will not be set in this case, causing a null
pointer dereference.
fixes#2144
Michael Stapelberg