openssh: add link to related nixos test

It'd certainly be better to have an openssh-specific integration test,
but it's not there yet and this is better than nothing.

pkgs/tools/networking/openssh/common.nix

@@ -22,6 +22,7 @@
 , withKerberos ? true
 , libkrb5
 , libfido2
+, nixosTests
 , withFIDO ? stdenv.hostPlatform.isUnix && !stdenv.hostPlatform.isMusl
 , linkOpenssl ? true
 }:
@@ -111,6 +112,10 @@ stdenv.mkDerivation rec {
     "sysconfdir=\${out}/etc/ssh"
   ];
 
+  passthru.tests = {
+    borgbackup-integration = nixosTests.borgbackup;
+  };
+
   meta = {
     description = "An implementation of the SSH protocol${extraDesc}";
     homepage = "https://www.openssh.com/";