Make test/gfx/at-file-ref consistent

The generated `result.pal` can change between platforms, it turns out
(due to "unstable sort" differences), and creates additional churn if
changing anything related to palette generation.

tl;dr this makes the test less flaky in the long run.

test/gfx/at-file-ref.flags

@@ -1,2 +1,2 @@
 -m
--c gbc:result.pal
+-c gbc:at-file-ref.pal