start-testsuite.sh: allow redefining TITAN_LIBRARY_PATH from env

I am not using Debian, so I always have to edit this file in order
to be able to run the testsuites.  Keep using default paths for
Debian, but allow redefining the TITAN_LIBRARY_PATH variable.

Change-Id: I3778a52697a182dbac39de6c18a053832ef78d93
1 file changed