GODT-1159 SMTP server not restarting after restored internet

- [x] write tests to check that IMAP and SMTP servers are closed when there
  is no internet
- [x] always create new go-smtp instance during listenAndServe(int)

internal/serverutil/server_test.go

@@ -0,0 +1,35 @@
+// Copyright (c) 2021 Proton Technologies AG
+//
+// This file is part of ProtonMail Bridge.
+//
+// ProtonMail Bridge is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// ProtonMail Bridge is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with ProtonMail Bridge.  If not, see <https://www.gnu.org/licenses/>.
+
+package serverutil
+
+import (
+	"testing"
+
+	"github.com/ProtonMail/proton-bridge/internal/serverutil/mocks"
+	"github.com/stretchr/testify/require"
+)
+
+func TestServerTurnOffAndOnAgain(t *testing.T) {
+	r := require.New(t)
+	s := mocks.NewTestServer(12321)
+
+	r.True(s.IsPortFree())
+
+	go ListenAndServe(s, s.EventListener)
+	s.RunServerTests(r)
+}