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?