RSpec: disable_monkey_patching + let! usage

[olleolleolle]

This PR is a formatting PR. Hopefully, this is easier to scan.

It uses two RSpec features:

• disable_monkey_patching setting
• let!, which is a an alternative to instance variables in before blocks

TODO:

• [ ] Harmonize ' into " for these files
• [ ] Avoid extra spaces

[kritik]

Cannot merge right now as code is outdated