Fix the lua standalone test

This fixes the subtest that checks if bcc-lua depends on
libluajit.
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>

tests/lua/test_standalone.sh

@@ -15,7 +15,8 @@ if [[ ! -x bcc-lua ]]; then
     exit 0
 fi
 
-if ldd bcc-lua | grep -q luajit; then
+LIBRARY=$(ldd bcc-lua | grep luajit)
+if [ $? -ne 0 -o -z "$LIBRARY" ] ; then
     fail "bcc-lua depends on libluajit"
 fi