test: Make the child process wait longer so it gets actually killed

Prior to this, depending on the load on jenkins, the test could be flaky and fail for the wrong reason [1]

• [1] https://jenkins.softwareheritage.org/job/DLDHG/job/tests-on-diff/263/console

Test Plan

tox ok

Also !125 (closed) based on this is no longer failing for the wrong reason.

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Migrated from D6040 (view on Phabricator)