Implement shed_test command.

- Don't test local stuff - test stuff published to shed.
  - Setup an on-the-fly Galaxy instance with `planemo.galaxy_config` module.
  - Update to actually configure shed artifacts.
  - Use `bioblend` to install the repository.
  - Run the tests (I think `run_tests.sh --installed` might be do this).
- Allow specifying a `shed_target`.
- Use `.shed.yml` to resolve id, etc....
