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/ds_file_test.go

@@ -1,8 +1,6 @@
 package test
 
 import (
-	"io/ioutil"
-	"log"
 	"testing"
 
 	"github.com/coredns/coredns/plugin/proxy"
@@ -49,8 +47,6 @@ func TestLookupDS(t *testing.T) {
 	}
 	defer i.Stop()
 
-	log.SetOutput(ioutil.Discard)
-
 	p := proxy.NewLookup([]string{udp})
 	state := request.Request{W: &mtest.ResponseWriter{}, Req: new(dns.Msg)}