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/ds_file_test.go

@@ -32,6 +32,7 @@ var dsTestCases = []mtest.Case{
 }
 
 func TestLookupDS(t *testing.T) {
+	t.Parallel()
 	name, rm, err := TempFile(".", miekNL)
 	if err != nil {
 		t.Fatalf("failed to created zone: %s", err)