tests: use simple mock smtp server instead of deprecated asyncore smtpd
test-patchbomb-tls.t would fail with:
.../hg/tests/dummysmtpd.py:6: DeprecationWarning: The asyncore module is deprecated and will be removed in Python 3.12. The recommended replacement is asyncio
import asyncore
.../hg/tests/dummysmtpd.py:8: DeprecationWarning: The smtpd module is deprecated and unmaintained and will be removed in Python 3.12. Please see aiosmtpd (https://aiosmtpd.readthedocs.io/) for the recommended replacement.
import smtpd
The recommended migration path to the standalone asiosmtpd would be overkill.
The tests do not need a full smtp server - we can just use a very simple mock
hack to preserve the existing test coverage.
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