[Go SDK] Validate Side Input behavior WRT windows

DoFns with Side inputs implicitly observe windows, as Side Inputs are scoped to a current window, a powerful feature of beam.

The ideal would be to you could try to unit test this in the exec package, creating a fake side input adapter (or use the real one), to more directly target the implementation. 

https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/core/runtime/exec/sideinput.go#L34 and exercising the ParDo code for Side Input handling directly.

https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/core/runtime/exec/pardo.go#L38 
Then it should be possible to bolster that with appropriate  mock windows to ensure that side inputs are configured correctly.

Altnernatively this behavior could be tested and validated with an integration test against real runners due to the complexity around Side Inputs making unit testing a challenge. (While side input code could be tested in that fashion, it's likely dramatically simpler to do the integration test.)

Some light tests with Side Inputs already exist, but they're purely in the Global Window. Add tests for non-global windows to ensure that Side Inputs are scoped correctly.

Integration test directory: https://github.com/apache/beam/tree/master/sdks/go/test/integration/primitives