Test sampled jobs from background migrations

What does this MR do and why?

Describe in detail what your merge request does and why.

This is one component of a first iteration in testing background migrations in the gitlab-com-database-testing pipeline (see gitlab-org/database-team/gitlab-com-database-testing#41 (closed)).

The code added here is not yet called anywhere in the gitlab project, but will be used by the database testing pipeline.

To use it, the testing pipeline will be modified to inject a fake post-migration with a timestamp of all 9s and the following code:

class TestBackgroundMigrations < Gitlab::Database::Migration[1.0]
    disable_ddl_transaction!
    def up
        Gitlab::Database::Migrations::TestBackgroundRunner.new.run_jobs(for_duration: 15.minutes) # or some other duration
    end
end

The testing pipeline already support for injecting patches, see here.

This will then run after all other migrations in the testing pipeline, and provide information about background migrations.

Later iterations will move this from a migration in the testing pipeline to a dedicated rake task, record data about individual background migration runs, then present that data in a more useful way.

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