core.db.cli: Add coverage and ensure `swh db *` works as expected

This explicitely checks the different scenario possible with the current subcommands (create, init-admin, init).

Using a subset of sql schema representation we have in our existing datasets.

Closes #2741 (closed)

Test Plan

tox

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Migrated from D4381 (view on Phabricator)

swh/core/db/tests/data/cli/0-superuser-init.sql

+create extension if not exists pgcrypto;