Switch to 'pytest' as a runner for unit tests

Pytest is much more user-friendly.

1. Better runner - colors, stacktraces, code snipets, hides "garbage" stdout/err/logger output.
2. Concise tests - just `def test(): assert ...` instead of junit-approach
3. Tons! of [useful plugins ](https://docs.pytest.org/en/latest/reference/plugin_list.html); ability to create bigflow-specific.

Proposal: add optional support of pytest.
Default test framework should remain
the same (unittest), but users might enable pytest via parameters to `bigflow.build.setup`.