Mostly finish pytest to Bats conversion

[hyperupcall]

This finalizes the conversion from Pytest to Bats, mostly so coverage can be calculated. Big shoutout to @vanpipy for making the original tests, which made things easier.

Code coverage is mentioned in the testing README.md. I got it to work with bashcov:

bashcov -- bats ./tests

coverage/index.html looks like this:

This is what opening a file looks like:

I wasn't able to get kcov to work, unfortunately. If anyone else is able to figure that out, I would appreciate it. But bashcov seems to work fine.

I updated CI to move the Bats testing to a separate job because it speeds things up, testing in parallel.

[spacewander]

@hyperupcall
Merged. Thanks!