jwack: _cheatfds error when run from toplevel make -j.

Also added a new unit test to confirm that 'make' behaviour works as
expected, with and without parallelism.

t/203-make/wipe-redo.sh

@@ -0,0 +1,13 @@
+vars=$(
+	env | {
+		IFS="="
+		while read key value; do
+			if [ "$key" != "${key#REDO}" ] ||
+			   [ "$key" != "${key#MAKE}" ]; then
+				echo "$key"
+			fi
+		done
+	}
+)
+echo "Wiping vars:" $vars >&2
+unset $vars