Changed tests from Mockito to Mockk

[freemanjp]

More idiomatic.

[codecov]

Codecov Report

Merging #140 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #140   +/-   ##
=========================================
  Coverage     72.49%   72.49%           
  Complexity      135      135           
=========================================
  Files            70       70           
  Lines           589      589           
  Branches        166      166           
=========================================
  Hits            427      427           
  Misses            1        1           
  Partials        161      161

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