cli/command/completion: add more unit-tests

[thaJeztah]

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

Had some of these stashed while I was working on some earlier changes; looks like I never pushed them

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.92%. Comparing base (21eea1e) to head (e1c472a).
Report is 28 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5533      +/-   ##
==========================================
+ Coverage   60.67%   60.92%   +0.25%     
==========================================
  Files         345      345              
  Lines       23491    23491              
==========================================
+ Hits        14252    14313      +61     
+ Misses       8266     8205      -61     
  Partials      973      973              

[laurazard]

Something else that this made me check/think about is that it'd be nice if the completion for commands that can take multiple arguments (such as docker rm [container] [container] didn't include arguments already added. I'll write a ticket

Thinking about this again, this is something that, if not handled by the shell, should be handled by the completions generated by cobra, not here.

[laurazard]

LGTM