Tests may succeed or fail depending on what's already stored in user's preferences

We store recent credential file locations, team/key IDs, payloads, etc. in a user's [`Preferences`](https://docs.oracle.com/javase/8/docs/api/java/util/prefs/Preferences.html). It appears that stored values can interfere with test results, and we should avoid letting a user's actual preferences/stored values interfere with unit tests.