This PR adds `gotestsum` to the nix dev shell. Running the tests without having`gotestsum` systemwide installed works now.
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/9169
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: Daniel Laguna <labellson@fastmail.com>
Co-committed-by: Daniel Laguna <labellson@fastmail.com>