Use absolute path to PYTHON and PERL, so test -x works properly in system tests

bin/tests/system/dnstap/tests.sh

@@ -499,7 +499,7 @@ if [ $ret != 0 ]; then echo_i "failed"; fi
 status=`expr $status + $ret`
 
 HAS_PYYAML=0
-if [ -n "$PYTHON" ] ; then
+if [ -x "$PYTHON" ] ; then
 	$PYTHON -c "import yaml" 2> /dev/null && HAS_PYYAML=1
 fi