Add widget tests for MyApp

[bizz84]

With the introduction of the onboarding screen (#41), the top-level widget switching logic would benefit from some tests.

This is to check that SignInPage, OnboardingPage or HomePage is returned depending on the application state.

[Abacaxi-Nelson]

i dont know if its related, but i had to signout from the Dashboard (first page of the app) because, when i was signing out from the profile page (the page was push on the stack (on top of dashboard)), the top-level widget was not updated (even if the code was working, messages log show that the good widget was return)....