tox.ini: Add swh.core[testing] requirement

This fixes the build with tox after recent changes in swh-core dependencies.

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Migrated from D4575 (view on Phabricator)

Build is green

Patch application report for D4575 (id=16260)

Rebasing onto cbd9c82c...

Current branch diff-target is up to date.

Changes applied before test

commit e8f5c81a3329eaa35d46289de313c8a064c1ab2e
Author: Antoine Lambert <antoine.lambert@inria.fr>
Date:   Tue Nov 24 14:20:54 2020 +0100

    tox.ini: Add swh.core[testing] requirement

See https://jenkins.softwareheritage.org/job/DLDBASE/job/tests-on-diff/339/ for more details.

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