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/metrics_test.go

@@ -2,7 +2,6 @@ package test
 
 import (
 	"io/ioutil"
-	"log"
 	"os"
 	"path"
 	"testing"
@@ -145,8 +144,6 @@ func TestMetricsAuto(t *testing.T) {
 	}
 	defer i.Stop()
 
-	log.SetOutput(ioutil.Discard)
-
 	// Write db.example.org to get example.org.
 	if err = ioutil.WriteFile(path.Join(tmpdir, "db.example.org"), []byte(zoneContent), 0644); err != nil {
 		t.Fatal(err)