mktemp needs 6 or more X's (fix #1000)

mktemp(1) isn't posix specified, but it's reasonable to use
mktemp(3) to implement it which requires 6 trailing X's in
the format

Thanks @sthen

tests/setup

@@ -29,7 +29,7 @@ clean () {
     fi
 }
 trap clean EXIT
-d=`mktemp -d -t jqXXXXX || true`
+d=`mktemp -d -t jqXXXXXX || true`
 if [ -z "$d" ]; then
     echo "Your OS does not support mktemp(1) -d" 1>&2
     exit 1