Fix the `Testing` docs

**Describe the bug**
Fix bugs in the `Testing` docs code: https://wasp.sh/docs/project/testing

1. This should be `!== 0` 
```ts
export function areThereAnyTasks(tasks: Task[]): boolean {
  return tasks.length === 0;
}
```
2. React component test example sets `value={task.isDone}` but then uses `toBeChecked()` test, which will fail. We should set `checked`.

There might be more.