arti: Add tests for the hss/hsc subcomands

This adds a handful of trycmd-based tests for the hsc and hss subcommands.

Some test cases are *.trycmd/*.md file, while others are in toml format. This is because the crate has limited support for eliding content in *md files (the *md tests support variables, but not [..] patterns).

I'm quite happy with trycmd so far. It makes it quite easy to add new tests, and requires little to no boilerplate.

Closes #1250 (closed)