Add tests to CLI

[matejcik]

let's not add hand-written test harnesses when there is a recommended way to do this

[matejcik]

To be more concrete, I don't want to have to run tests in a convoluted way, when right now you just type pytest and it runs what it needs. Any additional tests should also be invoked through pytest. And Click provides a nice facility for simple CLI testing, so that's the tool you should use if you want to implement tests for the CLI

[jonathancross]

@infokiller Can these be added to https://github.com/trezor/python-shamir-mnemonic/blob/master/test_shamir.py ?

[infokiller]

I'll look into adding the tests using click

[matejcik]

@jonathancross let's also not stuff everything into the same file, alright? :)