testsuite: fix "empty" linear-gradient compare test

We need to provide color stops to avoid rounding errors with different
shaders.

That makes the empty linear gradient somewhat less empty, but I think
it's the emptiest we can make it.

testsuite/gsk/compare/empty-linear-gradient.node

@@ -1 +1,10 @@
-linear-gradient { }
+linear-gradient {
+  stops: 0.2 red,
+         0.2 blue,
+         0.4 blue,
+         0.4 yellow,
+         0.6 yellow,
+         0.6 lime,
+         0.8 lime,
+         0.8 teal;
+}