I also agree with this analysis, and I will take it a step further. Not only did he want to test Pentangeli's reaction, he wanted Pentangeli, who was already uneasy that Michael came unnanounced, to be so shaken that he would give up on his idea to wipe out the Rosatos and make a deal with them as part of a larger plan to help Michael take his revenge against Roth.
But what could have been Frankie's reaction that he was testing?
"You know Mike, I know we're family and all, but you are a real SOB and I just wish to God that they would have plugged you."
or ...
"Well, Mike, ya know that this is the business we've chosen."
or ..., as he did say
"Mike, I almost died myself."