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/chaos_test.go

@@ -1,8 +1,6 @@
 package test
 
 import (
-	"io/ioutil"
-	"log"
 	"testing"
 
 	// Plug in CoreDNS, needed for AppVersion and AppName in this test.
@@ -24,8 +22,6 @@ func TestChaos(t *testing.T) {
 	}
 	defer i.Stop()
 
-	log.SetOutput(ioutil.Discard)
-
 	m := new(dns.Msg)
 	m.SetQuestion("version.bind.", dns.TypeTXT)
 	m.Question[0].Qclass = dns.ClassCHAOS