Makefile: Add a "make test" target.

I just wasted far too much time looking for a bug that wasn't actually
there only because I hadn't recompiled before running the test
suite. Now we can take advantage of actual dependency information to
force a rebuild for "make test".

Makefile.local

@@ -45,6 +45,10 @@ ifeq ($(shell cat .first-build-message),)
 endif
 endif
 
+.PHONY: test
+test: all
+	@./test/notmuch-test
+
 $(TAR_FILE):
 	git archive --format=tar --prefix=$(PACKAGE)-$(VERSION)/ HEAD > $(TAR_FILE).tmp
 	echo $(VERSION) > version