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>

plugin/pkg/healthcheck/log_test.go

@@ -0,0 +1,5 @@
+package healthcheck
+
+import clog "github.com/coredns/coredns/plugin/pkg/log"
+
+func init() { clog.Discard() }

plugin/pkg/log/log.go

@@ -10,6 +10,7 @@ package log
 
 import (
 	"fmt"
+	"io/ioutil"
 	golog "log"
 )
 
@@ -61,6 +62,9 @@ func Error(v ...interface{}) { log(err, v...) }
 // Errorf is equivalent to log.Printf, but prefixed with "[ERROR] ".
 func Errorf(format string, v ...interface{}) { logf(err, format, v...) }
 
+// Discard sets the log output to /dev/null.
+func Discard() { golog.SetOutput(ioutil.Discard) }
+
 const (
 	debug   = "[DEBUG] "
 	err     = "[ERROR] "