[SPARK-1341] [Streaming] Throttle BlockGenerator to limit rate of data consumption.

If we change this test, we should use [Scalatest's should matchers](http://www.scalatest.org/user_guide/using_matchers#creatingMatchersUsingLogicalOperators) so that the test failure message prints the actual values of the blocks that failed the assertion (or pass a failure message to `assert`).