Add a new passfailtest.

This tests that the target file isn't removed or changed if building fails.

t/passfailtest.do

@@ -0,0 +1,28 @@
+rm -f pleasefail
+redo passfail
+if [ ! -e passfail ]; then
+	echo "passfail should exist" >&2
+	exit 42
+fi
+PF1=$(cat passfail)
+touch pleasefail
+if redo passfail 2>/dev/null; then
+	echo "redo passfail should have failed" >&2
+	exit 42
+fi
+if [ ! -e passfail ]; then
+	echo "passfail should STILL exist" >&2
+	exit 42
+fi
+PF2=$(cat passfail)
+if [ "$PF1" != "$PF2" ]; then
+	echo "passfail changed even though it failed" >&2
+	exit 42
+fi
+rm -f pleasefail
+redo passfail || exit 43
+PF3=$(cat passfail)
+if [ "$PF1" = "$PF3" ]; then
+	echo "passfail did not change even though it passed" >&2
+	exit 42
+fi