Fix test TestProcessWorkflowRunTimeout_Pending (temporalio#7181)

## What changed?
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Fix test TestProcessWorkflowRunTimeout_Pending.

## Why?
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This test was flaky, and relying on the test suite `now` variable to be
very close to the mutable state start time. If the difference is less
than 1ms, I think the test passes.

## How did you test it?
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test? Checked in staging env? -->

## Potential risks
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change to production? -->

## Documentation
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stated in `docs/`? If significant
new behavior is added, have you described that in `docs/`? -->

## Is hotfix candidate?
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Author: rodrigozhou

service/history/timer_queue_standby_task_executor_test.go

@@ -1054,6 +1054,7 @@ func (s *timerQueueStandbyTaskExecutorSuite) TestProcessWorkflowRunTimeout_Pendi
 	// Flush buffered events so real IDs get assigned
 	mutableState.FlushBufferedEvents()
 
+	startTime := mutableState.GetExecutionState().GetStartTime().AsTime()
 	timerTask := &tasks.WorkflowRunTimeoutTask{
 		WorkflowKey: definition.NewWorkflowKey(
 			s.namespaceID.String(),
@@ -1062,7 +1063,7 @@ func (s *timerQueueStandbyTaskExecutorSuite) TestProcessWorkflowRunTimeout_Pendi
 		),
 		Version:             s.version,
 		TaskID:              s.mustGenerateTaskID(),
-		VisibilityTimestamp: s.now.Add(workflowRunTimeout),
+		VisibilityTimestamp: startTime.Add(workflowRunTimeout),
 	}
 
 	persistenceMutableState := s.createPersistenceMutableState(mutableState, completionEvent.GetEventId(), completionEvent.GetVersion())