Setting the result and outcomes of custom activity that was started in unit test, sets the result to null instead of the correct value and the outocmes array is empty

## Description
Using ActivityExecutionContext.Set() to set the result of my Activity started from an unit test, leaves the result as null. Also the activity outcomes is not set as well.

## Steps to Reproduce
1. Make a custom activity similar to the example provided below

```
[FlowNode(Outcomes.Success, Outcomes.Failed)]
public class MyActivity() : Activity
{
//input fields
protected override async ValueTask ExecuteAsync(ActivityExecutionContext context)
{
// code
context.CreateBookmark(new CreateBookmark
{
Payload = new WaitForActivityBookmark
{
Identifier = $"{context.Id}-{context.WorkflowExecutionContext.Id}"
},
Callback = ResumeAsync
});
}

private async ValueTask ResumeAsync(ActivityExecutionContext context)
{
Value = //code to get the value, this part was mocked and works fine

context.Set(Result,Value); //trying to set the result of my activity

if(Value.isSuccess)
{
await context.CompleteActivityWithOutcomesAsync("Success"); // trying to set outcome of the activity
}
else
{
await context.CompleteActivityWithOutcomesAsync("Failed"); // trying to set outcome of the activity
}
}
}

```
2. Make a unit test for this activity using Xunit. My unit test looks like this:

```
public MyActivityTest(ITestOutputHelper testOutputHelper)
{
_serviceProvider = new TestApplicationBuilder(
testOutputHelper)
.ConfigureServices(
serv =>
{
//other services...
serv.AddElsa(elsa =>
{
elsa.AddActivity();
elsa.UseDefaultAuthentication();

elsa.UseWorkflowsApi();

elsa.UseRealTimeWorkflows();

elsa.UseCSharp();
elsa.UseJavaScript();
elsa.UseLiquid();

elsa.UseHttp();
elsa.UseScheduling();

elsa.AddSwagger();
});
serv.AddElsaCore();
})
.Build();
}

[Fact(DisplayName = "MyActivity_Succeeds")]
public async Task MyActivity_Succeeds()
{
//making some input for my activity

var activity = new MyActivity()
{
//setting the required input
}

var workflowRunner = _serviceProvider.GetRequiredService();
var receivedResponse = await _serviceProvider.RunActivityAsync(activity); //starting the activity
var responseMock = new MyActivityResponse()
{
WorkflowIdentifier =
$"{receivedResponse.WorkflowState.ActivityExecutionContexts.FirstOrDefault()?.Id}-{receivedResponse.WorkflowState.Id}",
IsSuccess = true,
//other fields
}; // this will be received by the Value from ResumeAsync

var workflow = receivedResponse.Workflow;
var workflowState = receivedResponse.WorkflowState;
var bookmark = receivedResponse.WorkflowState.Bookmarks.Single();

var options = new RunWorkflowOptions()
{
BookmarkId = bookmark.Id,
ParentWorkflowInstanceId = workflowState.ParentWorkflowInstanceId,
ActivityId = bookmark.ActivityId,
ActivityInstanceId = bookmark.ActivityInstanceId,
ActivityNodeId = bookmark.ActivityNodeId,
};

// resume the activity based on the bookmark
receivedResponse = await workflowRunner.RunAsync(
workflow,
workflowState,
options,
cancellationToken: CancellationToken.None);
}
```

## Expected Behavior
The expected behavior would be to have the Value set as Result for my activity and for the Outcomes to not be empty.

## Environment
- **Elsa Package Version**: Elsa 3.1.2
- **Operating System**: Windows11

## Troubleshooting Attempts
Attempted to mock the Set() method but It gave the following error: System.NotSupportedException: 'Unsupported expression: x => x.Set(It.IsAny>(), It.IsAny(), "Result")
Non-overridable members (here: ActivityExecutionContext.Set) may not be used in setup / verification expressions.' Similar for SetResult() method.
Tried to use RunUntilEndAsync but got the same results.