Investigate slow cargo tests

[data-sync-user]

There’s some lengthier unit tests seen in CI that should be looked into, e.g.:

        PASS [   1.613s] tokenserver-common error::tests::sentry_event
        PASS [   2.065s] tokenserver-db models::tests::get_node_id
        PASS [   1.655s] tokenserver-db models::tests::heartbeat
        PASS [   1.691s] tokenserver-db models::tests::post_user
        PASS [   1.690s] tokenserver-db models::tests::replace_users
        PASS [   1.699s] tokenserver-db models::tests::test_allocation_is_not_allowed_to_backoff_nodes
        PASS [   1.707s] tokenserver-db models::tests::test_allocation_is_not_allowed_to_downed_nodes
        PASS [   1.675s] tokenserver-db models::tests::test_allocation_to_least_loaded_node
        PASS [   1.714s] tokenserver-db models::tests::test_correct_created_at_used_during_node_reassignment
        PASS [   1.697s] tokenserver-db models::tests::test_correct_created_at_used_during_user_retrieval
        PASS [   1.658s] tokenserver-db models::tests::test_get_spanner_node
        PASS [   1.729s] tokenserver-db models::tests::test_gradual_release_of_node_capacity
        PASS [   1.730s] tokenserver-db models::tests::test_node_allocation
        PASS [   1.695s] tokenserver-db models::tests::test_node_reassignment_and_removal
        PASS [   1.664s] tokenserver-db models::tests::test_node_reassignment_not_done_for_retired_users
        PASS [   1.716s] tokenserver-db models::tests::test_node_reassignment_when_records_are_replaced
        PASS [   1.679s] tokenserver-db models::tests::test_update_generation
        PASS [   1.673s] tokenserver-db models::tests::test_update_keys_changed_at

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