Complete the Side Input load testing suite for Go

The Go SDK Load Tests for Side inputs are incomplete, and probably misconfigured.

When they were originally authored, the SDK didn't have a side input cache, or map side inputs, but these could be included as part of the tests now.

https://github.com/apache/beam/blob/master/sdks/go/test/load/sideinput/sideinput.go

Dashboard: http://104.154.241.245/d/-E9aGlFGk/side-input-load-tests?orgId=1&var-processingType=batch&var-sdk=go

Go Dataflow jenkins config: https://github.com/apache/beam/blob/master/.test-infra/jenkins/job_LoadTests_SideInput_Go.groovy
Go Flink jenkins config: https://github.com/apache/beam/blob/master/.test-infra/jenkins/job_LoadTests_SideInput_Flink_Go.groovy

Imported from Jira [BEAM-14425](https://issues.apache.org/jira/browse/BEAM-14425). Original Jira may contain additional context.
Reported by: lostluck.