Add some black-box testing

[suve]

It would be useful to add some tests to the program. We could start with some black-box tests, i.e. ones that run on a fully compiled program and just check if it produces the expected output without crashing.

Possible action points:

• Add a compile-time symbol, or a program option (--stdin?), that allows to pass any input to the program, instead of performing the RPM query
• Think of some edge cases that might be worth testing