test: add cmpout to testlib

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5699060
diff --git a/test/run.go b/test/run.go
index 4d4488a..3ba35f9 100644
--- a/test/run.go
+++ b/test/run.go
@@ -238,6 +238,9 @@
 	action = strings.TrimSpace(action)
 
 	switch action {
+	case "cmpout":
+		action = "run" // the run case already looks for <dir>/<test>.out files
+		fallthrough
 	case "compile", "build", "run", "errorcheck":
 		t.action = action
 	default: