dovecot: add the dovecot NixOS test to passthru.tests

While we already had some test we might as well add the test for that
exact package to the tests attribute set. After all that should be what
(primarily) tests dovecot.

pkgs/servers/mail/dovecot/default.nix

@@ -89,5 +89,6 @@ stdenv.mkDerivation rec {
   };
   passthru.tests = {
     opensmtpd-interaction = nixosTests.opensmtpd;
+    inherit (nixosTests) dovecot;
   };
 }