Improved Beta Tester Deletion

What's Changed

• Switch to String types when specifying devices by @Blackjacx in #52
• Handle "Shutting Down" simctl status by @Blackjacx in #53
• Fix screenshot generation because of broken TestPlan parameter by @Blackjacx in #57
• Fix xcTestRunFile Name Generation by @Blackjacx in #58
• Fix beta tester deletion by @Blackjacx in #60

Full Changelog: 0.3.0...0.4.0