Improve EmbeddedEngine and AbstractConnectorTest

Currently the AbstractConnectorTest's start(...) methods return before the connector task is started, but this makes it difficult for tests that verify how a connector handles failures and problem situations (e.g., incomplete or invalid configuration, etc.). Part of the problem is that the EmbeddedEngine doesn't expose a way to for the start method to know when the task has been started. Additionally, it is possible for the EmbeddedEngine to fail to stop a connector under certain error conditions.