Use the cairo renderer for gtk tests for now

We haven't tracked down yet why the window size tests
fail with Vulkan, so use cairo for now.

testsuite/gtk/meson.build

@@ -80,6 +80,7 @@ foreach t : tests
               'GSETTINGS_BACKEND=memory',
               'GTK_CSD=1',
               'G_ENABLE_DIAGNOSTIC=0',
+              'GSK_RENDERER=cairo',
               'G_TEST_SRCDIR=@0@'.format(meson.current_source_dir()),
               'G_TEST_BUILDDIR=@0@'.format(meson.current_build_dir())
             ],