Fix batched migrations worker tests

What does this MR do and why?

!85528 (merged) introduced some new tests for the batched background migration workers, that are not aware for the newly added batched_nackground_migrations.gitlab_schema column. Because of this they are using the default value from the factory (gitlab_main), and causing failures when testing the CI worker.

This MR updates the shared examples to set gitlab_schema according to the worker being tested.

Resolve #362821 (closed).

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