Tests/LibWeb: Add ref test for text-shadow

Based on Base/res/html/misc/text-shadow.html demo page.

Tests/LibWeb/Ref/reference/text-shadow-ref.html

@@ -0,0 +1,9 @@
+<style>
+* {
+    margin: 0;
+}
+body {
+    background-color: white;
+}
+</style>
+<img src="./images/text-shadow-ref.png">