When a test has a `reftest-wait` class, we now wait for fonts to load
and then for 2 `requestAnimationFrame` callbacks. This matches the
behavior of the WPT runner and allows more imported WPT tests to work
correctly.
Now that headless mode is built into the main Ladybird executable, the
headless-browser's only purpose is to run tests. So let's move it to the
testing directory and rename it to test-web (a la test-js / test-wasm).
