Unit tests should support global init

[cmelchior]

Fallout from #3457

Testing this is non-trivial since the Context is now saved in a system-wide Singleton that remains between unit tests.

We need to refactor to allow the following:

• Add tests for Realm.init() not called
• Don't require all unit tests to specifically require setting this.

A solution I think will work is:

1. Introduce a new RealmUnitTestRunner that extends AndroidJUnit4 runner. It will be responsible for calling Realm.init(InstrumentationTarget.getTargetContext())

2. Add a protected Realm.resetInit() method that allows specific unit tests to reset the state if needed.