[SPARK-4053][Streaming] Made the ReceiverSuite test more reliable, by fixing block generator throttling

In the unit test that checked whether blocks generated by throttled block generator had expected number of records, the thresholds are too tight, which sometimes led to the test failing.
This PR fixes it by relaxing the thresholds and the time intervals for testing.

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #2900 from tdas/receiver-suite-flakiness and squashes the following commits:

28508a2 [Tathagata Das] Made the ReceiverSuite test more reliable