Update relate_failure_issue_spec.rb to check for issue due date

Per comment gitlab-org/ruby/gems/gitlab_quality-test_tooling!353 (comment 2808801838) . This task seems to belong to the cleaning up effort for Clean up product_group usage following feature_... (gitlab-org/quality&210) .

Right now we don't have a test case to check that due date is assigned to newly created failure issue. After a quick check, I found out that issue due date is determined using product_group here.

So in order to add a test case to check for issue due date, we also need to update def new_issue_due_date(test) method.