Run tests in parallel (#478)

Create a small speedup running the tests:

PASS
ok  	github.com/miekg/coredns/test	10.329s

PASS
ok  	github.com/miekg/coredns/test	6.079s

Skip the etcd ones. Doing the middleware/*/*_test ones doesn't yield
any speedup as these are still done on a per directory basis.

test/file_reload_test.go

@@ -13,6 +13,7 @@ import (
 )
 
 func TestZoneReload(t *testing.T) {
+	t.Parallel()
 	log.SetOutput(ioutil.Discard)
 
 	name, rm, err := TempFile(".", exampleOrg)