Add tests to Params in preparation for more refactoring

[dotasek]

Adds tests to Params, trying to cover as many common cases in translating args to ValidationContext as possible.

This will allow greater confidence when args are separated according to task.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 13.45%. Comparing base (585305e) to head (85936e6).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2197      +/-   ##
============================================
+ Coverage     13.42%   13.45%   +0.03%     
- Complexity    37344    37475     +131     
============================================
  Files          2460     2460              
  Lines        725666   725674       +8     
  Branches     212230   212230              
============================================
+ Hits          97390    97667     +277     
+ Misses       595006   594717     -289     
- Partials      33270    33290      +20     

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[grahamegrieve]

ok