Lint the test suite with shellcheck

Currently, we have some integration tests written in Bash. We do not run these under shellcheck yet, but might want to do that as part of our CI pipeline.

Another option: move these to Python or Haskell. They are getting a bit large, and adding tests currently involves a lot of boilerplate.

Locate the Bash integration tests and the repository's existing CI configuration. First determine how shellcheck can be run for those tests, then clarify whether rewriting them in Python or Haskell is part of this issue. Done means the agreed test scripts are checked by CI, with any migration scope decided explicitly.

Written by the indexing model from the issue text.