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.

Fixes builds [1] [2]

• [1] https://jenkins.softwareheritage.org/job/DLDHG/job/tests/793
• [2] https://jenkins.softwareheritage.org/job/DENV/job/tests/lastBuild/console

Similar to swh-loader-git!126 (closed)

Test Plan

tox

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Migrated from D3984 (view on Phabricator)