Tests/LibWeb: Add TransformStream transform callback test

This test proves the ability of TransformStream to execute to execute
caller supplied code in the transform callback that can transform
incoming chunks, and have access to TransformStreamDefaultController.

Tests/LibWeb/Text/expected/Streams/TransformStream-transform-callback.txt

@@ -0,0 +1,3 @@
+Done: false
+HELLO, WORLD!
+Done: true

Tests/LibWeb/Text/input/Streams/TransformStream-transform-callback.html

@@ -0,0 +1,22 @@
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        const {writable, readable} = new TransformStream({
+            transform(chunk, controller) {
+                controller.enqueue(chunk.toUpperCase());
+            }
+        });
+        const writer = writable.getWriter();
+        writer.write("Hello, world!");
+        writer.close();
+        const reader = readable.getReader();
+        reader.read().then(function processText({done, value}) {
+            println(`Done: ${done}`);
+            if (done)
+                return;
+
+            println(value);
+            reader.read().then(processText);
+        });
+    });
+</script>