LibWeb/Tests: Add basic test for ByteLengthQueuingStrategy

Author: https://github.com/shannonbooth
Commit: https://github.com/SerenityOS/serenity/commit/a92d94f4e9
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/ByteLengthQueuingStrategy-enqueue-desired-size.txt

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+5
+2
+-5
+-6

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

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+<script src="../include.js"></script>
+<script>
+    test(() => {
+        let controller;
+        new ReadableStream(
+            {
+                start(c) {
+                    controller = c;
+                }
+            },
+            new ByteLengthQueuingStrategy({ highWaterMark: 5 })
+        );
+
+        println(controller.desiredSize);
+        controller.enqueue(new Uint8Array([1, 2, 3]));
+        println(controller.desiredSize);
+        controller.enqueue(new Uint8Array([1, 2, 3, 4, 5, 6, 7]));
+        println(controller.desiredSize);
+        controller.enqueue(new Uint8Array([1]));
+        println(controller.desiredSize);
+    });
+</script>