[SPARK-20285][TESTS] Increase the pyspark streaming test timeout to 30 seconds

## What changes were proposed in this pull request?

Saw the following failure locally:

```
Traceback (most recent call last):
  File "/home/jenkins/workspace/python/pyspark/streaming/tests.py", line 351, in test_cogroup
    self._test_func(input, func, expected, sort=True, input2=input2)
  File "/home/jenkins/workspace/python/pyspark/streaming/tests.py", line 162, in _test_func
    self.assertEqual(expected, result)
AssertionError: Lists differ: [[(1, ([1], [2])), (2, ([1], [... != []

First list contains 3 additional elements.
First extra element 0:
[(1, ([1], [2])), (2, ([1], [])), (3, ([1], []))]

+ []
- [[(1, ([1], [2])), (2, ([1], [])), (3, ([1], []))],
-  [(1, ([1, 1, 1], [])), (2, ([1], [])), (4, ([], [1]))],
-  [('', ([1, 1], [1, 2])), ('a', ([1, 1], [1, 1])), ('b', ([1], [1]))]]
```

It also happened on Jenkins: http://spark-tests.appspot.com/builds/spark-branch-2.1-test-sbt-hadoop-2.7/120

It's because when the machine is overloaded, the timeout is not enough. This PR just increases the timeout to 30 seconds.

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #17597 from zsxwing/SPARK-20285.