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/metrics_test.go

@@ -16,6 +16,8 @@ import (
 	"github.com/miekg/dns"
 )
 
+// fail when done in parallel
+
 // Start test server that has metrics enabled. Then tear it down again.
 func TestMetricsServer(t *testing.T) {
 	corefile := `example.org:0 {