Fix flaky test_grab_ready_* tests
Hopefully? eg. see https://jenkins.softwareheritage.org/job/DSCH/job/tests-on-diff/341/console
Migrated from D5705 (view on Phabricator)
Hopefully? eg. see https://jenkins.softwareheritage.org/job/DSCH/job/tests-on-diff/341/console
Migrated from D5705 (view on Phabricator)