Ladybird+Tests/LibWeb: Add very basic text-only test harness

This allows us to create "text tests" in addition to "layout tests".
Text tests work the same as layout tests, but dump the document content
as text and exit upon receiving the window "load" event.

Tests/LibWeb/Text/input/basic.html

@@ -0,0 +1,9 @@
+<div id="out"></div><script>
+    function println(s) {
+        const out = document.getElementById("out");
+        out.appendChild(document.createTextNode(s + "\n"))
+    }
+
+    println("Well hello")
+    println("friends!")
+</script>