1. Add matchers for the `ProcessExit` exception class
2. Add ability to control the setup of the `AppContext` we use in
   `testUsingContext()`
3. Clean up the code that figures out the location of `Cache.flutterRoot`
   such that it works with `pub run test`. It previously only worked
   when the tests were invoked with standalone `dart`

`#3` above will also help unblock #7941