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
diff --git a/start-testsuite.sh b/start-testsuite.sh
index b9324bb..a1e753d 100755
--- a/start-testsuite.sh
+++ b/start-testsuite.sh
@@ -47,7 +47,7 @@
 
 # below is for the debian packages
 TTCN3_BIN_DIR="${TTCN3_BIN_DIR:-/usr/bin}"
-TITAN_LIBRARY_PATH="/usr/lib/titan:/usr/ttcn3/lib"
+TITAN_LIBRARY_PATH="${TITAN_LIBRARY_PATH:-/usr/lib/titan:/usr/ttcn3/lib}"
 LD_LIBRARY_PATH="$LD_LIBRARY_PATH:$SUITE_DIR:$TITAN_LIBRARY_PATH" "$TTCN3_BIN_DIR/ttcn3_start" $SUITE $CFG $TEST
 
 expected="$SUITE_DIR/expected-results.xml"