Fix flaky parallel design management uploads spec
What does this MR do and why?
Switch to non-transactional tests so that threads will not share the same DB connection. If they share the same connection, our transaction detection code would fail when the other thread is inside a transaction.
References
Related to #570457 (closed)
How to set up and validate locally
I can reproduce the flakiness pretty easily with just:
bin/rspec spec/graphql/mutations/design_management/upload_spec.rb:71MR acceptance checklist
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Edited by Heinrich Lee Yu