Add unit tests

[mittal-parth]

Hi @ggwpez!

Need a little assistance of yours here.

While testing, we should only be asserting from the top level and not the actual efficacy of the inner commands? What I mean is that if someone runs polkadot-dev-cli prdoc generate 111111 - we wont have to assert whether a file was created or not. We should only test if the command executed (or rather printed something on the terminal) correctly.

Thanks!