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

まず、Espresso の Idling システムと、この issue で説明されている同期動作を確認します。test API が View または Fragment をどのように待機するべきかを、タイムアウト条件と失敗条件を含めて定義します。非同期な UI の表示に対する、確定した設計と対応するカバレッジが揃えば完了です。

索引モデルが issue の本文から書いたものです。