BATS: `shellcheck`

[gaelicWizard]

Description

As shellcheck supports BATS syntax, turn it on and use it! And then fix all the tests!

Motivation and Context

This is chapter 3 of my BATS adventure. This is for the tests themselves, not the main code/libraries/plugins. This ensures that our tests are working properly.

NOTE: this does NOT enable "strict mode" in BATS, it's still just shellcheck.

How Has This Been Tested?

Lots and lots and lots of test/run...

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] My code follows the code style of this project.
• [x] If my change requires a change to the documentation, I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh.
• [x] I have added tests to cover my changes, and all the new and existing tests pass.

[gaelicWizard]

Ok! This one should be good to go. This just does shellcheck, not shfmt and not anything else. (I'll do shfmt in another PR.)