ParallelRequestsFixture is already tested in sync and async

[LukeButters]

Background

[SC-54458] The tests is already tested in sync and async. With two separate methods to avoid thread pool exhaustion.

How to review this PR

Quality :heavy_check_mark:

Pre-requisites

• [ ] I have read How we use GitHub Issues for help deciding when and where it's appropriate to make an issue.
• [ ] I have considered informing or consulting the right people, according to the ownership map.
• [ ] I have considered appropriate testing for my change.

[shortcut-integration[bot]]

This pull request has been linked to Shortcut Story #54458: Async Proxy + Ability for Tests to Test Sync and Async.