Fix flaky test StreamingListenerSuite "receiver info reporting"

As per the unit test log in https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/35754/

```
15/06/24 23:09:10.210 Thread-3495 INFO ReceiverTracker: Starting 1 receivers
15/06/24 23:09:10.270 Thread-3495 INFO SparkContext: Starting job: apply at Transformer.scala:22
...
15/06/24 23:09:14.259 ForkJoinPool-4-worker-29 INFO StreamingListenerSuiteReceiver: Started receiver and sleeping
15/06/24 23:09:14.270 ForkJoinPool-4-worker-29 INFO StreamingListenerSuiteReceiver: Reporting error and sleeping
```

it needs at least 4 seconds to receive all receiver events in this slow machine, but `timeout` for `eventually` is only 2 seconds.
This PR increases `timeout` to make this test stable.

streaming/src/test/scala/org/apache/spark/streaming/StreamingListenerSuite.scala

@@ -116,7 +116,7 @@ class StreamingListenerSuite extends TestSuiteBase with Matchers {
 
     ssc.start()
     try {
-      eventually(timeout(2000 millis), interval(20 millis)) {
+      eventually(timeout(30 seconds), interval(20 millis)) {
         collector.startedReceiverStreamIds.size should equal (1)
         collector.startedReceiverStreamIds(0) should equal (0)
         collector.stoppedReceiverStreamIds should have size 1