Hm. I'll chip in with an opinion based on surprises I got experimenting a
bit with the mix test task.
If I wanted to run tests across multiple files identified by line numbers,
I would naively write `mix test test/some_test.exs:15
test/some_test.exs:20`.
`mix test test/some_test.exs` will run
`mix test --only line:74 --only line:92
test/potion_web/plugs/persist_request_plug_test.exs` does work, but don't
you think it's a little confusing to allow `--only` more than once?
I'm cool with the `mix test
test/potion_web/plugs/persist_request_plug_test.exs:92:94:102` syntax.
Whatever
