Tests/LibWeb: Add ref test for CSS gradients

This has to cheat and use a screenshot but thanks to the "Take Full
Screenshot" feature of Ladybird, it is very easy to update this test.

The steps are documented in the test.

Tests/LibWeb/Ref/css-gradients-ref.html

@@ -0,0 +1,15 @@
+<style>
+  * {
+    margin: 0;
+  }
+  body {
+    background-color: white;
+  }
+</style>
+<!-- To rebase:
+  1. Open css-gradients.html in Ladybird
+  2. Resize the window just above the width of the largest gradient
+  3. Right click > "Take Full Screenshot"
+  4. Update the image below:
+-->
+<img src="./images/css-gradients-ref.png">