gitea: add passthru.tests

Reference nixos tests to execute on package upgrades.

pkgs/applications/version-management/gitea/default.nix

···
 , git, bash, gzip, openssh, pam
 , sqliteSupport ? true
 , pamSupport ? true
+, nixosTests
 }:
 
 with lib;
···
   '';
 
   goPackagePath = "code.gitea.io/gitea";
+
+  passthru.tests.gitea = nixosTests.gitea;
 
   meta = {
     description = "Git with a cup of tea";