Annotate stateful VR test in TestStreamTest with UsesStatefulParDo (related to #22472)

[mosche]

Annotate stateful VR test in TestStreamTest with UsesStatefulParDo for #22620 (#22472).

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[mosche]

R: @kennknowles

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[mosche]

R: @echauchot
R: @aromanenko-dev
(tiny PR to fix a VR test annotation)

[aromanenko-dev]

LGTM

[kennknowles]

FWIW I just meant to have this as a separate commit in the same PR. We do not require PRs to be squashed, so I could create a merge commit that would preserve them separately.