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).
Changed the usage from `add_libweb_test.py test_name.html` to
`add_libweb_test.py test_name.html test_type` with no default,
and supports automatically generating input/output files in the
right directories for test types Screenshot, Text, Ref, and Layout.
Co-authored-by: Sam Atkins <sam@ladybird.org>
