LibWeb/Tests: Add basic test for CountQueuingStrategy

Author: https://github.com/shannonbooth
Commit: https://github.com/SerenityOS/serenity/commit/b2a51e86e5
Pull-request: https://github.com/SerenityOS/serenity/pull/19555
Reviewed-by: https://github.com/kennethmyhra ✅
Reviewed-by: https://github.com/mattco98 ✅

Tests/LibWeb/Text/expected/Streams/CountQueuingStrategy-enqueue-desired-size.txt

@@ -0,0 +1,5 @@
+3
+2
+1
+0
+-1

Tests/LibWeb/Text/input/Streams/CountQueuingStrategy-enqueue-desired-size.html

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+<script src="../include.js"></script>
+<script>
+    test(() => {
+        let controller;
+        new ReadableStream(
+            {
+                start(c) {
+                    controller = c;
+                }
+            },
+            new CountQueuingStrategy({ highWaterMark: 3 })
+        );
+
+        println(controller.desiredSize);
+        controller.enqueue('Hello, friends!');
+        println(controller.desiredSize);
+        controller.enqueue('Enqueues being counted...');
+        println(controller.desiredSize);
+        controller.enqueue('By CountQueuingStrategy');
+        println(controller.desiredSize);
+        controller.enqueue('!');
+        println(controller.desiredSize);
+    });
+</script>