alertmanager: Added nixosTests to passthru.tests

pkgs/servers/monitoring/prometheus/alertmanager.nix

@@ -3,6 +3,7 @@
 , buildGoModule
 , fetchFromGitHub
 , installShellFiles
+, nixosTests
 }:
 
 buildGoModule rec {
@@ -39,6 +40,8 @@ buildGoModule rec {
     installShellCompletion amtool.zsh
   '';
 
+  passthru.tests = { inherit (nixosTests.prometheus) alertmanager; };
+
   meta = with lib; {
     description = "Alert dispatcher for the Prometheus monitoring system";
     homepage = "https://github.com/prometheus/alertmanager";