Makefile: run tests sequentially

Our tests are not designed to be run in parallel, for example there are
multiple tests in disparate packages all trying to run textnextnethttp
which will compete for access to port 9191, we shouldn't let tests run
in parallel.

Makefile

@@ -58,12 +58,12 @@ endif
 test: check-cert
 ifeq "$(TRAVIS)" "true"
 ifdef DARWIN
-	sudo -E go test -v $(ALL_PACKAGES)
+	sudo -E go test -p 1 -v $(ALL_PACKAGES)
 else
-	go test $(TEST_FLAGS) $(BUILD_FLAGS) $(ALL_PACKAGES)
+	go test -p 1 $(TEST_FLAGS) $(BUILD_FLAGS) $(ALL_PACKAGES)
 endif
 else
-	go test $(TEST_FLAGS) $(BUILD_FLAGS) $(ALL_PACKAGES)
+	go test -p 1 $(TEST_FLAGS) $(BUILD_FLAGS) $(ALL_PACKAGES)
 endif
 ifneq "$(shell which lldb-server)" ""
 	@echo