In an autobuilder environment, there will typically be no hardware GPU
available, so Mesa will fall back from hardware to Zink to software
rendering. Unfortunately, Zink logs to stderr during loading if no
hardware GPUs are available. This particular test asserts that stderr
has desired contents, which means Zink's extra output causes the test
to fail. We can bypass this by disabling use of Zink.
Resolves: https://gitlab.gnome.org/GNOME/gtk/-/issues/6478
Signed-off-by: Simon McVittie <smcv@debian.org>
