## What changes were proposed in this pull request?
It is kind of annoying that `SharedSQLContext.sparkConf` is a val when overriding test cases, because you cannot call `super` on it. This PR makes it a function.

## How was this patch tested?
Existing tests.

Author: Herman van Hovell <hvanhovell@databricks.com>

Closes #17705 from hvanhovell/SPARK-20410.