a11y: Add testing API

a11y: Add testing API

We want to test the accessibility API, as well as the implementation
inside each widget. For that, we should expose an API that lets us
verify that a GtkAccessible has a given role, as well as a given
property.

The API follows the pattern of other GTest API:

 - a macro to assert that a condition is respected
 - a function that prints out the error message in case of failure