Tests/LibWeb: Add some text tests for 'parsing a legacy color value'

I was not aware of this framework back when implementing this back in
bc54560e59. Add in some basic tests for
this now that we are compliant with the specification.

Tests/LibWeb/Text/input/css/legacy-color-value.html

@@ -0,0 +1,24 @@
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        function checkColor(color) {
+            document.body.bgColor = color;
+            const computedStyle = getComputedStyle(document.body);
+            const bgcolor = computedStyle.backgroundColor;
+            println(`'${color}' => ${bgcolor}`);
+        }
+
+        for (color of [
+            "red",
+            "#408080",
+            "transparent",
+            " GreeN 	",
+            "cafe",
+            "",
+            "deadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeef", // longer than 128 chars of hex
+            "#emoji above U+FFFF 🙃",
+        ]) {
+            checkColor(color);
+        }
+    });
+</script>