Re-adds test target to the Makefile

The test target existed before, this is to provide a legacy interface to
allow easy testing for downstream Docker CE.

Without this we would need separate Makefiles/Jenkinsfiles for releases
past 17.07. Later on this target could also be used to test both unit
tests and integration tests at the same time.

Signed-off-by: Eli Uriegas <eli.uriegas@docker.com>

docker.Makefile

@@ -46,6 +46,9 @@ clean: build_docker_image
 test-unit: build_docker_image
 	docker run --rm $(ENVVARS) $(MOUNTS) $(DEV_DOCKER_IMAGE_NAME) make test-unit
 
+.PHONY: test
+test: test-unit test-e2e
+
 # build the CLI for multiple architectures using a container
 .PHONY: cross
 cross: build_cross_image