test: move custom WHATWG URL tests into separate files

To enable automatic update of WPT, move all our custom
WHATWG URL tests that are not present in the upstream into
files starting with `test-whatwg-url-custom-`, so it's easier
to identify test cases that can be upstreamed and test cases
that should be rolled into our repo (possibly with automation).

PR-URL: https://github.com/nodejs/node/pull/22442


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>