Makefile: adjust the test rule to be more useful

Run quick tests before the slow tests.

Change-Id: I52d0d9a72de416201745eff992d84d72409cecae
Reviewed-on: https://go-review.googlesource.com/c/playground/+/177477
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

Makefile

diff --git a/Makefile b/Makefile
index 3fbb2ed..4fe2ee7 100644
--- a/Makefile
+++ b/Makefile
@@ -1,8 +1,11 @@
-.PHONY: update-deps docker test
+.PHONY: docker test
 
-docker: Dockerfile
-	docker build -t playground .
+docker:
+	docker build -t golang/playground .
 
-test: docker
-	go test
-	docker run --rm playground test
+test:
+	# Run fast tests first: (and tests whether, say, things compile)
+	GO111MODULE=on go test -v
+	# Then run the slower tests, which happen as one of the
+	# Dockerfile RUN steps:
+	docker build -t golang/playground .