Tests: Set `network_id` instead of `site_id` in `WP_UnitTest_Factory_For_Blog` defaults.

Follow-up to [47011], [47013].

See #47195.
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git-svn-id: https://core.svn.wordpress.org/trunk@46814 1a063a9b-81f0-0310-95a4-ce76da25c4cd