Clean up tests logging (#1979)

* Clean up tests logging

This cleans up the travis logs so you can see the failures better.

Older tests in tests/ would call log.SetOutput(ioutil.Discard) in
a haphazard way. This add log.Discard and put an `init` function in each
package's dir (no way to do this globally). The cleanup in tests/ is
clear.

All plugins also got this init function to have some uniformity and kill
any (future) logging there in the tests as well.

There is a one-off in pkg/healthcheck because that does log.

Signed-off-by: Miek Gieben <miek@miek.nl>

* bring back original log_test.go

Signed-off-by: Miek Gieben <miek@miek.nl>

* suppress logging here as well

Signed-off-by: Miek Gieben <miek@miek.nl>

test/etcd_cache_test.go

@@ -4,8 +4,6 @@ package test
 
 import (
 	"context"
-	"io/ioutil"
-	"log"
 	"testing"
 
 	"github.com/coredns/coredns/plugin/etcd/msg"
@@ -33,7 +31,6 @@ func TestEtcdCache(t *testing.T) {
 	defer ex.Stop()
 
 	etc := etcdPlugin()
-	log.SetOutput(ioutil.Discard)
 
 	var ctx = context.TODO()
 	for _, serv := range servicesCacheTest {