Add tests for cmdline.py

[jugmac00]

• Add test for printing help when cli is called with no args
• Add test for the overview cli option

This is one PR (of many) to get test coverage to 100%, see #92

[codecov]

Codecov Report

Merging #93 (e1cba1c) into master (805e1b3) will increase coverage by 0.49%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
+ Coverage   93.33%   93.83%   +0.49%     
==========================================
  Files           7        8       +1     
  Lines        1291     1363      +72     
  Branches      117      132      +15     
==========================================
+ Hits         1205     1279      +74     
+ Misses         64       63       -1     
+ Partials       22       21       -1     

Impacted Files	Coverage Δ
tnefparse/cmdline.py	95.83% <0.00%> (ø)
cmdline.py	95.83% <0.00%> (+6.94%)	⬆️

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