Set up a basic integration test suite

[rochlefebvre]

Part of #7

Big PR! Wherein I:

• rip out the boilerplate rspec stuff. We'll use rubygems-style Test::Unit stuff in case parts of this plugin ever make it into rubygems itself;
• Add test-unit, webmock, and update openid_connect to 1.3 to avoid an expired root cert issue in a transitive dependency;
• Use Gem::UserInteraction as the abstraction for IO instead of $stdout;
• Add a bunch of helpers for stubbing web requests via webmock;
• Add one happy path integration test for gem sign;
• Add a default rake task: test;
• Add a rudimentary GH workflow for the default rake task: test.

So now we finally have test~s~!