[BEAM-14406] Drain test for SDF truncation in Go SDK

[riteshghorse]

Adds a drain test for SDF truncation during drain added in #17432 . It runs only on Dataflow as of now but it is filtered from it because drain has to be triggered manually from the UI.

Screenshots of truncation in action:

closes #21681

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[riteshghorse]

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R: @youngoli for final approval

[riteshghorse]

R: @lostluck

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[lostluck]

I'll note that we can add a programatic bit that could drain the job automatically, in particular for this test. But that's a fair amount of work, since we'd need to provide access to the JobID, and the Dataflow client, via some kind of "running job" wrapper.

[lostluck]

LGTM