Add Testing library command, heavily refactor

[stevehanson]

• Adds testing library command to install testing library
• Adds several utilities, such as copyTemplateDirectory
• Refactors some of the existing commands to use copyTemplateDirectory and copyTemplate
• Updates mock fs-extra and tests to work with the new template copy and other utilities
• Tested that I can generate a new app with the CLI then cd into the app and run yarn test:all and everything passes (ESLint, Prettier, TSC, tests). I'd like to soon add tests that do this against the latest version of Expo (and eventually RN init)