Write tests for the module and its settings

Problem/Motivation

Would be great to have tests to ensure the module works as expected. Shouldn't be too hard, as the module is not that complicated. So I'll tag this as novice task.

The tests should include

1. Create dummy content, like nodes or taxonomy terms
2. Placing a block of this type for a certain menu
3. Use the default block settings and ensure the results appear as expected
4. Ensure the block is present and shows up as expected
5. Adjust all settings one by one and ensure things go as expected

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes