[TEST] Introduced mockito-scala testing library

[stanislavlyalin]

Overview

This PR is not for a merge, but for discussion.

Added mockito-scala library for generating mock objects for unit tests.

Only one demo test has been changed. The rest of the tests are commented out for now.

Please make sure that this PR:

• [x] is at most 200 lines of code (excluding tests),
• [x] meets RChain development coding standards,
• [x] includes tests for all added features,
• [x] has a reviewer assigned,
• [x] has all commits signed.

Bors cheat-sheet:

• bors r+ runs integration tests and merges the PR (if it's approved),
• bors try runs integration tests for the PR,
• bors delegate+ enables non-maintainer PR authors to run the above.

[tgrospic]

For now it seems that Mockito cannot give us easier way to test effectful code. Let's keep it on hold until further research.