Quite

"This is a litmus test I use for how close I am with a friend. If s/he doesn't tell me anything bad about their life, I assume we're not very good friends."

By that standard though, I should think most people would play it pretty close to the chest. There's also a couple odd thoughts involved in that

1) What about people who don't tell you anything but the bad parts of their lives? I would say you might be useful for them in the same way that a psychologist is, but at some point, maybe something good happened? Most people are pretty good at mixing this up at least a little. But people who don't tell stories, not as much. Such folks tend not to need other people to validate that they had fun doing something. What they need is advice and support.

2) How does one cross the dividing line from a person who tells you only good things or has positive shared experiences to being a trusted source of advice or confidence of dealing with much lesser and bad or painful experiences? It would seem like this might a line that many people might not want crossed and might prefer the more casual nature of associations. It can be kind of a Rubicon point in a friendship of any kind to confess your sins and struggles and weaknesses instead of the happiness.