arti: Rewrite hand-written tests using `trycmd`

This replaces the hand-rolled arti keys list test suite with a set of trycmd tests. These should be easier to extend and understand, and usually also double as documentation (note: trycmd is the foundation of most of the tests we have for the arti CLI, so this shouldn't be too controversial).

Ideally, all the CLI tests would be written this way, but there still a few other modules in cli_tests that probably need to be ported to trycmd at some point.

Note: some of the tests I'm removing in 3a40f260 don't have a trycmd equivalent in arti/tests/testcases/keys. In my opinion, this is perfectly fine, because the old tests were unnecessarily testing all the possible configurations involving 1-2 keystores (one arti, and one C Tor): it is not the responsibility of the CLI tests to cover all these permutations (they should be tested in tor-keymgr).