directory config & cli need integration tests

[nothingmuch]

So far I have only done manual testing of the config & cli stuff because such tests would be boilerplate heavy and mostly testing functionality due to be removed soon.

After #915 is merged we should revisit this and consider whether or not to retain the environment variable configuration, as well as add a CLI option to specify the config file and the revisit the decision whether to use the dirs trait to find the config file.

[spacebear21]

The directory is now defunct in favor of payjoin-mailroom, and I think that CLI and config is comparitively lightweight enough that we can get away without testing those for now.