Unit tests should assert output to stderr on errors

[dwreeves]

This is already included in #161, but it is not being formally unit-tested. The current unit-tests do not distinguish between stderr and stdout.

This relates to #164.

This may be related to #163. It's unclear how syrupy works. If it's anything like insta then moving to syrupy makes life easier, but I imagine it's a little more complicated than that.

[sforet]

claim