testsuite: Add a forgotten test

The test existed in git but wasn't hooked up. So let's do that by:

1. Adding it to the build
2. Adapting it a bit so rounding errors really don't trigger (as the
   original commit claimed they shouldn't).
3. Re-renaming it because this was actually about 3d gradients

testsuite/gsk/compare/linear-gradient-3d-nocairo.node

@@ -1,5 +1,5 @@
 clip {
-  clip: 0 -25 50 50;
+  clip: 1 -25 50 50;
   child: transform {
     transform: perspective(100) rotateY(45);
     child: linear-gradient {

testsuite/gsk/meson.build

@@ -71,6 +71,7 @@ compare_render_tests = [
   'inset-shadow-multiple',
   'invalid-transform',
   'issue-3615',
+  'linear-gradient-3d-nocairo',
   'linear-gradient-nonorthogonal-scale',
   'linear-gradient-with-64-colorstops',
   'mask',