BUG: Do not use Slicer Launcher for Unix tests.

When run with valgrind during the CTest check, every test gets about 10 false
positives from the shell executable.

Removing the use of the launcher script on unix.

@christopher @aylward @jcfr @chrismullins look good?

A Slicer Extension Experimental build still passes:

  http://open.cdash.org/buildSummary.php?buildid=3162194