Add widget tests for MyApp

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.

Start by locating the MyApp widget and the existing widget-test setup. Exercise the application states that select SignInPage, OnboardingPage, and HomePage, then verify the returned page for each state; done means all three switching paths are covered by widget tests.

Written by the indexing model from the issue text.