I disagree. Helping a stranger is easy (once you get past the awkward "why are you helping me? You don't even know me." part). The real challenge is helping someone you don't particularly like.
I just disagreed with a prophet, didn't I?
Perhaps which scenario is the "real test" depends on exactly what you're testing. If we're testing how much we love our fellowmen, helping a stranger is the real test because it means you saw a random person who needed help and your first response was "that person could use my help. I'm going to go help them." That shows a good deal of love for someone, even when you don't know them.
If what you're testing is how committed you are to helping others, even when you don't want to, then perhaps the "real test" is helping someone you'd rather not help. You don't need to love a person in order to help them. Just ask a sales clerk. Sometimes, you are or feel obligated to help someone, and that sense of duty convinces you to help them, despite the absence of brotherly love. Helping someone doesn't necessarily prove that you have charity for them.
However, I can see how if you neither like nor dislike someone because you don't really know them yet, and if you have an opportunity to help them, though you're under no obligation to, then whether or not you help them would be based almost entirely on how much charity you have for them. Helping someone you don't like isn't charity. Loving someone you don't know is.