requirements-test: Add missing asynctest dependency

This fixes the builds [1] [2]

Note that i don't see that dependency packaged in debian so at some point, some work will be needed for it ;)

• [1] https://jenkins.softwareheritage.org/job/DTSCN/job/tests/297/console
• [2] https://jenkins.softwareheritage.org/job/DENV/job/tests/lastBuild/console

Test Plan

tox

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Migrated from D4331 (view on Phabricator)