Synchronization capabilities: allow to wait for a View or Fragment to exist

Espresso should allow to wait for a View or Fragment to appear (with a timeout).

The "Idling system" makes a lot of assumptions about the implementation details of the app, and makes difficult to test the **UI** of an asynchronous app without having to instrument async operations in the app, which is simply not realistic for modern apps where everything is async (or should be anyway). This makes the success of the test depend on instrumentation code spread around the codebase instead of just the actual behaviour of the app and the code of the test itself.

In general, testing the UI should not _require_ knowledge about the implementation details of the backend.

It would help if the test API allowed to wait for a View or Fragment to appear, with a timeout or other failure conditions.

Many thanks

Comienza revisando el sistema Idling de Espresso y el comportamiento de sincronización descrito en este issue. Define cómo debe esperar la test API a una View o un Fragment, incluidas las condiciones de timeout y de fallo; el trabajo estará terminado cuando exista un diseño consensuado y la cobertura correspondiente para la aparición asíncrona de la UI.

Escrito por el modelo de indexación a partir del texto del issue.