Use new mail package instead of an unmintained one (#32682)

Resolve #18664

services/mailer/sender/main_test.go

@@ -0,0 +1,14 @@
+// Copyright 2019 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package sender
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/models/unittest"
+)
+
+func TestMain(m *testing.M) {
+	unittest.MainTest(m)
+}