--- a/Makefile
+++ b/Makefile
@@ -37,7 +37,7 @@
 %.test: %.test.log
 	@echo "========================================"
 	@echo "checking output of test $*"
-	@diff $< $*.test.expected || cat $<
+	@diff -u $*.test.expected $< || cat $<
 
 %.test.log: %.test.txt
 	@echo "========================================"