Integration testing of Pipeline drain in Go SDK

[damccorm]

Add an integration test for pipeline drain feature BEAM-11106 added in https://github.com/apache/beam/pull/17432. 

Imported from Jira BEAM-14406. Original Jira may contain additional context.
Reported by: riteshghorse.

[damccorm]

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[riteshghorse]

@damccorm

[damccorm]

FWIW, this limitation should be addressed by #21719

[riteshghorse]

Great, thanks!

[lostluck]

I'll note that we can probably make a special handle for Dataflow jobs so we can programmatically trigger the drain after we have the job confirmed as running.

It wouldn't be able to run via ptest, but that's fine.