Fix cross-database modification in runner spec factories

What does this MR do and why?

When instantiating a ci_runner using the factory it was previously also instantiating a project, group and user and then when it called save! these would all be persisted in a single transaction along with the ci_runner but this violates our "cross-database transactions" check https://docs.gitlab.com/ee/development/database/multiple_databases.html#removing-cross-database-transactions .

As such we can instead call create when instantiating these relations to ensure they are saved before the runner transaction.

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