test: doctest: rest/docs/domains.rst: Sort owners by user_id
The Mailman REST API at `/3.0/domains/<domain>/owners` does NOT ensure
order of owner entries in a response. Thus the doctest in
`src/mailman/rest/docs/domains.rst` sometimes fails.
Failed CI jobs for example:
* [pgsql-39 (#
6523349845) · Jobs · GNU Mailman / Mailman Core · GitLab](https://gitlab.com/mailman/mailman/-/jobs/
6523349845)
* [pgsql-310 (#
6493325803) · Jobs · GNU Mailman / Mailman Core · GitLab](https://gitlab.com/mailman/mailman/-/jobs/
6493325803)
* [pgsql-311 (#
6075425574) · Jobs · GNU Mailman / Mailman Core · GitLab](https://gitlab.com/mailman/mailman/-/jobs/
6075425574)
Closes #1150