Tests/LibWeb: Verify XHR.send() throws when called twice

This verifies that XHR.send() throws an Invalid State Error when called
twice.

Author: kennethmyhra

Tests/LibWeb/Text/expected/XHR/XMLHttpRequest-send-flag-already-set.txt

@@ -0,0 +1 @@
+PASS

Tests/LibWeb/Text/input/XHR/XMLHttpRequest-send-flag-already-set.html

@@ -0,0 +1,16 @@
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        const INVALID_STATE_ERR = 11;
+        try {
+            const xhr = new XMLHttpRequest();
+            xhr.open("GET", "data:text/plain,", true);
+            xhr.send();
+            xhr.send();
+        }
+        catch (e) {
+            if (e.code === INVALID_STATE_ERR)
+                println("PASS");
+        }
+    });
+</script>