Execute tests for should_fail directory

Makefile

@@ -16,6 +16,8 @@ all: pdf
 test: pdf
 	LD_LIBRARY_PATH=$(HAMMER_LIB) sh -c \
 	'for x in t/*.pdf; do ./pdf "$$x" >/dev/null && echo OK: "$$x"; done'
+	LD_LIBRARY_PATH=$(HAMMER_LIB) sh -c \
+	'for x in t/should_fail/*.pdf; do ./pdf "$$x" >/dev/null ; done'
 	@true
 
 pdf: $(SOURCES)