Get the WebSocket URL from the /api/links endpoint instead of the `websocketUrl`
configuration in app.html. This ensures that the client uses the correct
WebSocket endpoint for the h API service it is currently talking to, which may
be different than the default when  `services` configuration is specified.

When updating the tests, several had to be reworked to be less sensitive
to the number of microtask ticks in between certain events.

This depends on the h change in https://github.com/hypothesis/h/pull/7253.