requirements-test.txt: Add arrow dependency to fix tox build
Side effect of rDSCH3c8707599ae1cc721c5ee80b6def30e2374725b6.
The arrow package was installed in the tox venv through swh-scheduler dependency. It is no more a scheduler dependency but should be installed through swh-core[http] dependency. Unfortunately pip will install swh-core first and will skip swh-core[http] due to its old resolver behavior. So we need to explicitely add the arrow package as test requirement.
Migrated from D4681 (view on Phabricator)