LibWeb: Add tests for background-position

The css-backgrounds.html ref test file only tests the most basic usage
of this, which failed to catch some regressions, so let's add a more
extensive test. :^)

Tests/LibWeb/Ref/reference/css-background-position-ref.html

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