Even with all the efforts to cleanup the temp directories created by
unit tests, Spark leaves a lot of garbage in /tmp after a test run.
This change overrides java.io.tmpdir to place those files under the
build directory instead.

After an sbt full unit test run, I was left with > 400 MB of temp
files. Since they're now under the build dir, it's much easier to
clean them up.

Also make a slight change to a unit test to make it not pollute the
source directory with test data.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #6674 from vanzin/SPARK-8126 and squashes the following commits:

0f8ad41 [Marcelo Vanzin] Make sure tmp dir exists when tests run.
643e916 [Marcelo Vanzin] [MINOR] [BUILD] Use custom temp directory during build.