Use feature-test to detect feature support in system tests

Previously, there were two different ways to detect feature support.
Either through an environment variable set by configure in conf.sh, or
using the feature-test utility.

It is more simple and consistent to have only one way of detecting the
feature support. Using the feature-test utility seems superior the the
environment variables set by configure.

bin/tests/system/synthfromdnssec/tests.sh

@@ -751,7 +751,7 @@ do
 	echo_i "Skipping XML statistics checks"
    fi
 
-   if [ ${HAVEJSONSTATS} ] && [ -x "${CURL}" ] ; then
+   if $FEATURETEST --have-json-c && [ -x "${CURL}" ] ; then
 	echo_i "getting JSON statisistcs for (synth-from-dnssec ${description};) ($n)"
 	ret=0
 	json=json.out$n