Use FEATURETEST variable instead of a path

feature-test tool location needs to be determined by its associated
variable; otherwise, the tool is not found on Windows:

    setup.sh: line 22: ../feature-test: No such file or directory

bin/tests/system/upforwd/setup.sh

@@ -18,7 +18,7 @@ copy_setports ns1/named.conf.in ns1/named.conf
 copy_setports ns2/named.conf.in ns2/named.conf
 copy_setports ns3/named.conf.in ns3/named.conf
 
-if ../feature-test --enable-dnstap
+if $FEATURETEST --enable-dnstap
 then
 	cat <<'EOF' > ns3/dnstap.conf
 	dnstap-identity "ns3";