Expose SetContinuedAsNewRunID in the TestWorkflowEnvironment

[lminaudier]

What was changed

This extends the TestWorkflowEnvironment to expose a new SetContinuedAsNewRunID API to set the workflow under test ContinuedAsNewRunID field in its WorkflowInfo.

Why?

When building some workflows ("actors" like ones are a good example), you sometimes want to be able to distinguish when a workflow is starting and when it's being continued as new. This allows you to execute different behavior like restoring some state from an external system.

One way to check if your current workflow execution has been continued as new is to inspect the workflow info ContinuedAsNewRunID field. When it's set, then you now you are not starting fresh. It is nice because you don't need any special field in your workflow inputs to detect such cases.

Example:

func MyWorkflow(ctx workflow.Context, input string) error {
    if workflow.GetInfo(ctx).ContinuedAsNewRunID == "" {
        var v string
        err := workflow.ExecuteActivity(ctx, MyActivity).Get(ctx, &v)
        if err != nil {
            return err
        }
        input = v
    }

    s = selector.NewSelector()
    s.AddReceive(..., func() {
        c.Receive(ctx, &v)
        input = v
    })
    s.AddReceive(...)
    s.Select(ctx)

    return workflow.NewContinueAsNewError(ctx, MyWorkflow, input)
}

When testing such workflows though, you may want to test behavior only after a first ContinueAsNew call has been made. The easiest way to do that is to setup the test environment to fill up the workflow info ContinuedAsNewRunID field.

This is what this commit is doing, it extends the
TestWorkflowEnvironment to expose a SetContinuedExecutionID field to an arbitrary value.

This way in workflow unit tests, we can skip the code paths we don't care about and don't resort to mocking activities and child workflows for nothing leading to easier to implement and easier to read tests.

Checklist

1. Closes

N/A

2. How was this tested:

I only added a unit test (so far).

3. Any docs updates needed?

We probably want to rely on Godocs for that API.

---

Thoughts?

[Quinn-With-Two-Ns]

Thanks for the contribution