Fix race in FanOutStreamingEngineWorkerHarnessTest

[scwhittle]

Multiple assignments can be racily collapsed causing the verification to block.

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[scwhittle]

R: @Abacn

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control. If you'd like to restart, comment assign set of reviewers

[scwhittle]

assign set of reviewers

[github-actions]

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R: @jrmccluskey added as fallback since no labels match configuration

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[damccorm]

Thanks!