ci: fix ShellCheck linting for BATS tests

[georglauterbach]

Description

Make lint.sh lint BATS tests again.

Type of change

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

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (README.md or the documentation under docs/)
• If necessary I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[casperklein]

Make lint.sh lint BATS tests again.

A short summary why it was broken (the reason) would have been nice. The new solution looks good, but also the previous one (haven't checked why it wasn't working).

[georglauterbach]

Sorry for the lack of explanation. The crux was: the directory containing the tests changed, and the linting script wasn't adjusted. This PR takes care of the directory problem and also of some minor code smells.