Changed tests from Mockito to Mockk

[freemanjp]

More idiomatic.

[codecov]

Codecov Report

Merging #250 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #250   +/-   ##
=========================================
  Coverage     83.99%   83.99%           
  Complexity       76       76           
=========================================
  Files            16       16           
  Lines           481      481           
  Branches         64       64           
=========================================
  Hits            404      404           
  Misses           33       33           
  Partials         44       44

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 34df447...aedb007. Read the comment docs.