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

bin/tests/system/conf.sh.in

@@ -106,12 +106,12 @@ XMLLINT=@XMLLINT@
 XSLTPROC=@XSLTPROC@
 
 # PERL will be an empty string if no perl interpreter was found.
-PERL=@PERL@
+PERL=$(command -v "@PERL@")
 
 # Windows process management leave empty
 PSSUSPEND=
 
-PYTHON=@PYTHON@
+PYTHON=$(command -v "@PYTHON@")
 
 #
 # Determine if we support various optional features.

bin/tests/system/digdelv/tests.sh

@@ -62,7 +62,7 @@ KEYDATA="$(< ns2/keydata sed -e 's/+/[+]/g')"
 NOSPLIT="$(< ns2/keydata sed -e 's/+/[+]/g' -e 's/ //g')"
 
 HAS_PYYAML=0
-if [ -n "$PYTHON" ] ; then
+if [ -x "$PYTHON" ] ; then
 	$PYTHON -c "import yaml" 2> /dev/null && HAS_PYYAML=1
 fi
 

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