Redesign tests + 3.0 support

[davetron5000]

This adds support for Ruby 3.0 by removing Aruba and reorganizing the tests. Namely:

• Matrixed build on Circle CI against all supported MRI Rubies
• All tests ported to Minitest and modernized/simplified
• Clean Test and all that is removed
• Aruba and Cucumber is gone and replaced with much simpler integration tests. These don't cover GLI quite as well, but given that everything has been working, I'll add more coverage when bugs are addressed or features are added. The pain of keeping them working over the years is not worth it
• The scaffolding removes cucumber and is slightly better, but still kinda crappy
• Moved gli to exe/gli to be consistent with new Bundler conventions