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/server_test.go

@@ -8,6 +8,7 @@ import (
 
 // Start 2 tests server, server A will proxy to B, server B is an CH server.
 func TestProxyToChaosServer(t *testing.T) {
+	t.Parallel()
 	corefile := `.:0 {
 	chaos CoreDNS-001 miek@miek.nl
 }