Makefile: exit non-zero for a failed test

Makefile

@@ -8,9 +8,12 @@ install:
 	cp -r zsh-syntax-highlighting.zsh highlighters $(SHARE_DIR)
 
 test:
+	@result=0
 	@for test in highlighters/*; do \
 		if [ -d $$test/test-data ]; then \
 			echo "Running test $${test##*/}"; \
 			zsh tests/test-highlighting.zsh "$${test##*/}"; \
+			: $$(( result |= $$? )); \
 		fi \
 	done
+	@exit $$result