[SPARK-14936][BUILD][TESTS] FlumePollingStreamSuite is slow

https://issues.apache.org/jira/browse/SPARK-14936

## What changes were proposed in this pull request?

FlumePollingStreamSuite contains two tests which run for a minute each. This seems excessively slow and we should speed it up if possible.

In this PR, instead of creating `StreamingContext` directly from `conf`, here an underlying `SparkContext` is created before all and it is used to create  each`StreamingContext`.

Running time is reduced by avoiding multiple `SparkContext` creations and destroys.

## How was this patch tested?

Tested on my local machine running `testOnly *.FlumePollingStreamSuite`

Author: Xin Ren <iamshrek@126.com>

Closes #12845 from keypointt/SPARK-14936.