I have had drinks with friends and friends of friends in bars, pubs and beverage rooms in UK, Canada and US. I am almost 70 years old. I have never asked for a drink, I have never been asked for one. If I saw this happen, I would assume that the asker either wanted to have a favour done for them because they were feeling low or was out of money. I would not suspect that it was some sort of test. I would expect the response to be buying the drink, making a joke about the request or avoiding further conversation (or maybe all of them). I am used to people buying drinks for one another in some situation but not asking for a drink.
In my experience, people are by and large not testing; they have good will towards others; and they like company. This includes Aspergers and NTs. Why start out suspicious?
Then maybe it's a generation thing. I am over 50. La, what curious customs these young things invent!
Followup to: Do you have High-Functioning Asperger's Syndrome?
LW reader Madbadger uses the metaphor of a GPU and a CPU in a desktop system to think about people with Asperger's Syndrome: general intelligence is like a CPU, being universal but only mediocre at any particular task, whereas the "social coprocessor" brainware in a Neurotypical brain is like a GPU: highly specialized but great at what it does. Neurotypical people are like computers with measly Pentium IV processors, but expensive Radeon HD 4890 GPUs. A High-functioning AS person is an Intel Core i7 Extreme Edition - with on-board graphics!
This analogy also covers the spectrum view of social/empathic abilities, you can think about having a weaker social coprocessor than average if you have some of the tendencies of AS but not others. You can even think of your score on the AQ Test as being like the Tom's Hardware Rating of your Coprocessor. (Lower numbers are better!).
If you lack that powerful social coprocessor, what can you do? Well, you'll have to run your social interactions "in software", i.e. explicitly reason through the complex human social game that most people play without ever really understanding. There are several tricks that a High-functioning AS person can use in this situation: