Docs: add unit test example

[yanyongyu]

fix #138

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 17.83%. Comparing base (16e6eb9) to head (a09e9d2).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #142   +/-   ##
=======================================
  Coverage   17.83%   17.83%           
=======================================
  Files        4846     4846           
  Lines      223563   223563           
=======================================
  Hits        39864    39864           
  Misses     183699   183699           

Flag	Coverage Δ
unittests	17.83% <ø> (ø)

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