Sometimes it's useful to be able to run tests located in specific
directories and/or files, let's enable this in `utils/build-script`
using `--test-paths` option which accepts a list of viable test locations.
Resolves: rdar://problem/32004487
* Added new OptionalTrueAction and OptionalFalseAction classes to the arguments module.
* Fixed missing return statement in help-test generator.
* Renamed all the `skip_build_*` names to the more simple and affirmative `build_*`.
* Renamed all the `skip_test_*` names to the more simple and affirmative `test_*`.
* Added documentation to the OptionalTrueAction and OptionalFalseAction classes.
* Implemented a (mostly) comprehensive test suite for the argument parser (ab)using metaclasses to dynamically generate unit-tests for each valid argument and preset.
* Fixed failing defaults test for build_ninja.
* Added new HelpOption and IgnoreOption classes for generating tests.
* Converted default value test into individual generated tests which should give better output in case the default tests fail.
