Fix nightly test command

[mgax]

Another fix for the nightly CI:

• Run tests using pytest, like tox.ini does.
• Only search for tests under tests/, so we don't pick up Wagtail's testsuite.

[mgax]

I've manually triggered a nightly workflow and it completed successfully: https://github.com/wagtail/wagtail-newsletter/actions/runs/9125935214.