testsuite: Disable test on cairo

With the --repeat version of this test, Cairo needs to draw partially
clipped glyphs. However, there's a bug in Cairo where it doesn't account
for the subpixel positioning when clipping, so the glyphs get cut off at
the edge.

This is filed as https://gitlab.freedesktop.org/cairo/cairo/-/issues/821

testsuite/gsk/meson.build

@@ -142,7 +142,7 @@ compare_render_tests = [
   'stroke-opacity',
   'stroke-with-3d-contents-nogl-nocairo',
   'subpixel-positioning',
-  'subpixel-positioning-hidpi-nogl',
+  'subpixel-positioning-hidpi-nogl-nocairo',
   'text-color-mix',
   'text-glyph-lsb',
   'text-mixed-color-nocairo',