test: Increase test coverage, documentation updates, typo fixes

[jlilja]

Another PR to address some stuff I found here and there :slightly_smiling_face:

I added some more information about the make testall command. Firstly it was to document for myself and understand how it works. But if you think that it could be useful for future contributors to understand, then I'd gladly have it merged as well.

I also found in the docs/data-sources/groups.md file that a typo had been introduced. So I updated that to have the right data resource name.

I was also trying to learn how to write tests for existing code already, and that led me to improve some test coverage.

Once this is merged, then I can submit a PR for https://github.com/jlilja/terraform-provider-pihole/pull/4, sorry for the clunkiness of it all :sweat_smile: