Global spark configuration changes are not maintained during Maven tests

Related to this issue: https://github.com/holdenk/spark-testing-base/issues/187

I want to increase spark.driver.maxResultSize; not just for testing but globally. I have it set in `conf/spark-default.conf` as

`spark.driver.maxResultSize 2g`

When I start a spark-shell or spark-submit a job; I can see that this is reflected in my Environment variables at http://:4040. However, when I run a `mvn test` using spark-testing-base, I get

```
[Stage 25:> (0 + 4) / 4]
18/08/14 17:24:16 ERROR TaskSetManager: Total size of serialized results of 3 tasks (1150.1 MB) is bigger than spark.driver.maxResultSize (1024.0 MB)
18/08/14 17:24:16 ERROR TaskSetManager: Total size of serialized results of 4 tasks (1533.5 MB) is bigger than spark.driver.maxResultSize (1024.0 MB)
```

This is the normal default spark.driver.maxResultSize. I'm can't determine if spark-testing-base is supposed to be using the configuration settings at `conf/spark-default.conf` or not, so I don't know if this is expected behavior or not. I don't see anywhere where a SparkConf object would override my conf file.

This is related to the post linked at the top, where you have two solutions for the problem I have. But, I'd like to know if changing the `conf/spark-default.conf` is another valid solution or not.

Start with conf/spark-default.conf and the spark-testing-base behavior exercised by mvn test; compare those settings with the reported spark-shell and spark-submit environment. Review the related issue #187 for context, then determine whether the configured spark.driver.maxResultSize should be honored during tests and document or verify the expected behavior.

Written by the indexing model from the issue text.