*: detect leaky goroutines, fix leaks

gexpect.Interact leaks. This adds ReadLine method to wait for the leaky
goroutine to accept an EOF.

Fixes https://github.com/coreos/etcd/issues/4258.

Reference: https://github.com/coreos/etcd/pull/4261#issuecomment-174198945.

integration/migration_test.go

@@ -17,10 +17,12 @@ package integration
 import (
 	"os/exec"
 	"testing"
+
+	"github.com/coreos/etcd/pkg/testutil"
 )
 
 func TestUpgradeMember(t *testing.T) {
-	defer afterTest(t)
+	defer testutil.AfterTest(t)
 	m := mustNewMember(t, "integration046", false)
 	cmd := exec.Command("cp", "-r", "testdata/integration046_data/conf", "testdata/integration046_data/log", "testdata/integration046_data/snapshot", m.DataDir)
 	err := cmd.Run()