[chore][chloggen] Run tests end-to-end

[djaglowski]

This PR increases test coverage by executing cobra commands directly. It also some new test cases and further validates expected outputs and errors.

[codecov]

Codecov Report

Patch coverage: 70.94% and project coverage change: +0.93 🎉

Comparison is base (db501f8) 60.69% compared to head (3a76020) 61.63%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #369      +/-   ##
==========================================
+ Coverage   60.69%   61.63%   +0.93%     
==========================================
  Files          48       48              
  Lines        2315     2330      +15     
==========================================
+ Hits         1405     1436      +31     
+ Misses        758      743      -15     
+ Partials      152      151       -1     

Impacted Files	Coverage Δ
chloggen/cmd/update.go	58.00% <58.00%> (+6.97%)	⬆️
chloggen/cmd/validate.go	68.42% <68.42%> (+21.36%)	⬆️
chloggen/cmd/new.go	78.78% <79.31%> (+17.49%)	⬆️
chloggen/cmd/root.go	92.00% <94.73%> (+38.66%)	⬆️

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