tests: give the full path to the softsig test helper program

tests: give the full path to the softsig test helper program

In order to start the softsig test helper properly,
the full path of this program is necessary.

FIXES 132246

Change-Id: I4e9ff1e62a0b82078338eeaf0d4368ac1b35dccc
Reviewed-on: http://gerrit.openafs.org/11977
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>