Add stake module tests

[dadamu]

Description

Related to #25

Checklist

• Targeted PR against correct branch.
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Wrote unit tests.
• Re-reviewed Files changed in the Github PR explorer.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@03d0c53). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #207   +/-   ##
=========================================
  Coverage          ?   67.08%           
=========================================
  Files             ?       46           
  Lines             ?     1826           
  Branches          ?        0           
=========================================
  Hits              ?     1225           
  Misses            ?      541           
  Partials          ?       60           

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