I am uncertain to what degree they are testing search results from Google, but on some web browsers I am seeing Yahoo! organics and ads powered by Bing & in other browsers I am seeing Yahoo! organics and ads powered by Google. Here are a couple screenshots.
Comparing The SERPs
Notable differences between the versions:
top ad color
top ad favicon
clickable ad area
right of each ad near URL
once in gray above all ads
ad URL redirect
ad units above organics
ad star rating color
Yahoo! verticals like Tumblr & Answers
mixed into organic results
not mixed in
footer "powered by Bing" message
When the Google ads run on the Yahoo! SERPs for many keywords I am seeing many of the search arbitrage players in the top ads. Typically these ads are more commonly relegated to Google.com's right rail ad positions.
The Google Yahoo! Search Backstory
Back in 2008 when Yahoo! was fighting to not get acquired they signed an ad agreement with Google, but it was blocked by the DOJ due to antitrust concerns. Unless Google loses Apple as a search partner, they are arguably more dominant today in general web search than they were back in 2008. Some have argued apps drastically change the way people search, but Google has went to great lengths to depreciate the roll of apps & suck people back into their search ecosystem with features baked into Google Now on tap & in-app keyword highlighting that can push a user from an app into a Google search result.
A little over a year ago Yahoo! launched Gemini to begin rebuilding their own search ad network, starting with mobile. In their Q1 report, RKG stated "Among advertisers adopting Gemini, 36% of combined Bing and Yahoo mobile traffic was served by Yahoo in March 2015."
When Yahoo! recently renewed their search deal with Microsoft, Yahoo! was once again allowed to sell their own desktop search ads & they are only required to give 51% of the search volume to Bing. There has been significant speculation as to what Yahoo! would do with the carve out. Would they build their own search technology? Would they outsource to Google to increase search ad revenues? It appears they are doing a bit of everything - some Bing ads, some Yahoo! ads, some Google ads.
Bing reports the relative share of Yahoo! search ad volume they deliver on a rolling basis: "data covers all device-types. The relative volume (y-axis) is an index based on average traffic in April, therefore it is possible for the volume to go above 1.0. The chart is updated on a weekly basis."
If Yahoo! gives Google significant share it could create issues where users who switch between the different algorithms might get frustrated by the results being significantly different. Or if users don't care it could prove general web search is so highly commoditized the average searcher is totally unaware of the changes. The latter is more likely, given most searchers can't even distinguish between search ads and organic search results.