Auto-clearing in tests

[henryiii]

Drops app.reset() in favor of automatic clearing if needed (parse() can now be used multiple times natively without extra work, even if an error is thrown and caught). A bug with the arg setting and ordering has been fixed; it was hidden in the tests due to a wrong placement of app.reset(). If you need to manually clear an app, app.clear() will do exactly what app.reset() used to do (consistent name with opt->clear()).

app.reset() was only intended for use in tests, so no DEPRECATED form provided.

[codecov]

Codecov Report

Merging #141 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #141   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines        1686   1690    +4     
=====================================
+ Hits         1686   1690    +4

Impacted Files	Coverage Δ
include/CLI/App.hpp	100% <100%> (ø)	⬆️

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