tests: Don't check the number of created 'person' objects.
They are an implementation detail of storage backends; and get_stats() will stop returning the 'person' count in the next version of swh-storage-core.
Additionally, swh-loader-core!363 (closed) changes the values of these counters.
Fixes [1]
Test Plan
tox
Migrated from D3978 (view on Phabricator)