SPARK-1181. 'mvn test' fails out of the box since sbt assembly does not necessarily exist

The test suite requires that "sbt assembly" has been run in order for some tests (like DriverSuite) to pass. The tests themselves say as much.

This means that a "mvn test" from a fresh clone fails.

There's a pretty simple fix, to have Maven's test-compile phase invoke "sbt assembly". I suppose the only downside is re-invoking "sbt assembly" each time tests are run.

I'm open to ideas about how to set this up more intelligently but it would be a generally good thing if the Maven build's tests passed out of the box.