It's a test case I wouldn't read too much into it. Maybe it's evidence of a massive anti-trust conspiracy at google, but it could very well be because it's the first domain that came to the programmer's mind at the time.

I wasn't aware of this, but it still seems like a thread worth pulling on. You're assuming, right? The reason I ask is that using any third-party company seems inappropriate. Even more so when Google has plenty of domains of its own to test against. Even more so when it is against a media/advertising company. And again, even more so against a company that changed from Google to Bing to power their search function. It seems to be an inappropriate or poor choice, doesn't it?

There's no smoking gun here, but I don't think that concern might be dismissed out of hand. It might be good to see what Yahoo's take on this. This could even evolve into participation by the US Attorney General. I'd like to know more, either way. Like if Yahoo was independently added to the list at a later date, or if it was there from the start?

The functionality is the functionality: it targets the header to Google sites. If there's a legal issue it really stands or falls there, not on the presence of another company's domain in the tests. There's nothing Yahoo-specific about what Chrome is actually doing.

I've long seen it almost as a tradition to use yahoo for things like testing if the internet is working, e.g. "ping yahoo.com". I suspect this isn't much more than that.

It's an arbitrary test string, not evidence of evil intent. A sufficiently uncharitable interpretation can make anyone's writing look evil. It's not so.