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_test.go

@@ -3,6 +3,7 @@ package test
 import "testing"
 
 func TestTempFile(t *testing.T) {
+	t.Parallel()
 	_, f, e := TempFile(".", "test")
 	if e != nil {
 		t.Fatalf("failed to create temp file: %s", e)