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/log_test.go

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