WeeklyRoundupEmailNotificationTestCase fails when run in CI, with "Connection Refused"

[alexdunnjpl]

🐛 Describe the bug

core.actions.test.roundup_test.WeeklyRoundupEmailNotificationTestCase fails when run in CI

📜 To Reproduce

Steps to reproduce the behavior:

1. Disable test skip
2. Push feature branch to CI

🕵️ Expected behavior

WeeklyRoundupEmailNotificationTestCase tests should pass, assuming they pass when tested locally

🩺 Test Data / Additional context

https://github.com/NASA-PDS/doi-service/actions/runs/3055258403/jobs/4928111199#step:6:744

⚙️ Engineering Details

Failure appears related to unavailability of the smtpd DebuggingServer, but this is confusing because another test relies on the same functionality, but does not fail. First thought is that if the tests are run in parallel, only the first server will be able to instantiate correctly (on the given port), but this is not the case afaik. Still worth checking out.