[SPARK-14391][LAUNCHER] Fix launcher communication test, take 2.

There's actually a race here: the state of the handler was changed before
the connection was set, so the test code could be notified of the state
change, wake up, and still see the connection as null, triggering the assert.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #12785 from vanzin/SPARK-14391.