[SPARK-17534][TESTS] Increase timeouts for DirectKafkaStreamSuite tests

**## What changes were proposed in this pull request?**
There are two tests in this suite that are particularly flaky on the following hardware:

2x Intel(R) Xeon(R) CPU E5-2697 v2  2.70GHz and 16 GB of RAM, 1 TB HDD

This simple PR increases the timeout times and batch duration so they can reliably pass

**## How was this patch tested?**
Existing unit tests with the two core box where I was seeing the failures often

Author: Adam Roberts <aroberts@uk.ibm.com>

Closes #15094 from a-roberts/patch-6.