test: increase unit test coverage to 80%

[Wwwsylvia]

1. Increase unit test coverage to 80%
2. Set Codecov threshold to 80%

Resolves: #779

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.15%. Comparing base (dff5628) to head (43165c0).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #876      +/-   ##
==========================================
+ Coverage   76.00%   80.15%   +4.15%     
==========================================
  Files          63       63              
  Lines        6043     6043              
==========================================
+ Hits         4593     4844     +251     
+ Misses       1068      860     -208     
+ Partials      382      339      -43     

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[shizhMSFT]

LGTM