feat: Add {testthat} test suite; first few tests

[datawookie]

Hi @jeroen, after tinkering with {gpg} a while ago I been working with the package a lot and it's really useful. I thought that it would make the overall package more robust (and facilitate any further development) if there was a test suite. So I've started to work on that, putting together the first few tests. Didn't want to go too far down this rabbit hole until I was sure that you were open to the idea. So submitting this PR early, with the intention of collaborating and adding more tests over time. Thanks, Andrew.

[jeroen]

Why is it loading stringi?

[datawookie]

Good point. Initial implementation used {stringi} to generate random strings. However, I remembered from last time that you are sensitive to the number of dependencies, so I implemented custom function to do this. Just forgot to not load {stringi}. Fixed now.