Add minimal (internal) tests

tests/common/server.nix

@@ -0,0 +1,13 @@
+{...}: {
+  imports = [./../../mailsystem];
+  config = {
+    virtualisation.memorySize = 1024;
+    mailsystem = {
+      enable = true;
+
+      roundcube.enable = false;
+      rspamd.webUi.enable = false;
+      certificateScheme = "selfsigned";
+    };
+  };
+}

tests/internal.nix

@@ -0,0 +1,40 @@
+{pkgs, ...}:
+pkgs.nixosTest {
+  name = "internal";
+  nodes.machine = {...}: {
+    imports = [./common/server.nix];
+    mailsystem = {
+      fqdn = "mail.example.com";
+      domains = ["example.com"];
+      accounts = {};
+      vmailUserName = "vmail";
+      vmailGroupName = "vmail";
+      vmailUID = 5000;
+    };
+  };
+  testScript = {nodes, ...}: let
+    pkgs = nodes.machine.nixpkgs.pkgs;
+  in ''
+    machine.start()
+    machine.wait_for_unit("multi-user.target")
+
+    with subtest("imap is only available via port 993 and is encrypted"):
+        machine.wait_for_closed_port(143)
+        machine.wait_for_open_port(993)
+        machine.succeed(
+            "echo | ${pkgs.openssl}/bin/openssl s_client -connect localhost:993 | grep 'New, TLS'"
+        )
+
+    with subtest("smtp is only available via port 465 and is encrypted"):
+        machine.wait_for_closed_port(587)
+        machine.wait_for_open_port(465)
+        machine.succeed(
+            "echo | ${pkgs.openssl}/bin/openssl s_client -connect localhost:465 | grep 'New, TLS'"
+        )
+
+    with subtest("`postfix check` succeeds"):
+        machine.succeed(
+            "${pkgs.postfix}/bin/postfix check"
+        )
+  '';
+}