Implement half of tests in Bats

[hyperupcall]

This supersedes #1170. As mentioned in the issue that the PR links to, the testing suite is being converted from Pytest to Bash, mostly due to Pytest's poor support for Bash codecov.

I changed the formatting for files in bats/ because they use a lot of heredocs for improved readability of the stdout content assertions. According to the Bash manual (emphasis mine):

If the redirection operator is <<-, then all leading tab characters are stripped from input lines and the line containing delimiter. This allows here documents within shell scripts to be indented in a natural fashion.

This adds the first half of tests just to get things started. The suite can be ran with:

bats ./tests

In another PR, I'll rewrite the other half of tests, add documentation, and eventually remove the pytest stuff.

For a more detailed explanation of functions and commands (for those not as familiar with Bats), #1170 contains very verbose details in comments

[spacewander]

Could we add a job in the CI to run the bats test? For now, we can run both py and shell until everything is done.

[vanpipy]

Great! pls replace the ci

git-extras/.github/workflows/ci.yml

Line 56 in fdaca2c

run: |

with the bats, that makes everything done easily.