Update tests

[jarrodldavis]

Probot's authors have made it easier to write tests for plugins, so probot-gpg should update applicable tests to match. Here's an example of updated tests in the official autoresponder plugin.

Additionally, more cases (where some or all commits aren't verified) need to be tested in the full integration suite.

Furthermore, now that more options are being added to the app, the tests are getting annoyingly repetitive. A data-driven approach should be take to generate test cases instead of duplicating test code.