Write DB testing output files to a directory

What does this MR do?

Adjust the instrumented DB migration task to account for the changes in gitlab-org/database-team/gitlab-com-database-testing!25 (merged). The output file is written to a directory, which we then artifact in the testing pipeline.

!60020 (merged) is a follow up to this MR which will write query logs to the directory so that they can be artifacted.

Related to gitlab-org/database-team/team-tasks#144 (closed).

Does this MR meet the acceptance criteria?

Conformity

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