Get rid of Mockito, and use Hypermocker instead.

[podusowski]

I want to get rid of Mockito, and use this crate instead. Once it can handle all Walkers' use cases, I will create a new repo for it.

I did not wanted to create yet another HTTP mocking library, but in Mockito, I could not deffer a response to the incoming request, which was a must if I wanted to test concurrent downloads properly.

Originally posted by @podusowski in #121 (comment)

[podusowski]

#126