Allow only existing FKs to be exempted

What does this MR do and why?

This MR makes sure that only existing, concrete foreign keys can be listed in the allowed_cross_database_foreign_keys array in spec/lib/gitlab/database/no_cross_db_foreign_keys_spec.rb spec.

This new assertion would prevent leftover entries in that array. For example, we would remove the foreign key via a migration, but would forget to remove it from the allowed_cross_database_foreign_keys array.

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