Changes to Makefile to run integration test on MacOS

When running integration test on MacOS, hit an error because of an old
docker image for antrea/openvswitch:2.13.0. Pulling that docker image
explicitly which is used as base image for test container image.

Makefile

@@ -105,6 +105,7 @@ docker-test-unit: $(DOCKER_CACHE)
 .PHONY: docker-test-integration
 docker-test-integration:
 	@echo "===> Building Antrea Integration Test Docker image <==="
+	@docker pull antrea/openvswitch:2.13.0
 	@docker build -t antrea/test -f build/images/test/Dockerfile .
 	@docker run --privileged --rm \
 		-e "GOCACHE=/tmp/gocache" \