tasks: Simplify implementation and add tests for listed origins

Recent changes in swh-scheduler add new parameters to the celery tasks
produced from swh.scheduler.model.ListedOrigin instances.

So ensure to handle any new parameters by not hardcoding the expected
ones in task signatures.

Remove unsafe use of unnamed task parameters.

Add new tests checking task parameters produced from ListedOrigin
instances do no raise error when attempting to create a hg loader.

Related to T4187