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/etcd_test.go

@@ -31,7 +31,7 @@ func etcdMiddleware() *etcd.Etcd {
 
 // This test starts two coredns servers (and needs etcd). Configure a stubzones in both (that will loop) and
 // will then test if we detect this loop.
-func TestEtcdStubForwarding(t *testing.T) {
+func TestEtcdStubLoop(t *testing.T) {
 	// TODO(miek)
 }