[SPARK-8126] [BUILD] Make sure temp dir exists when running tests.

If you ran "clean" at the top-level sbt project, the temp dir would
go away, so running "test" without restarting sbt would fail. This
fixes that by making sure the temp dir exists before running tests.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #6805 from vanzin/SPARK-8126-fix and squashes the following commits:

12d7768 [Marcelo Vanzin] [SPARK-8126] [build] Make sure temp dir exists when running tests.