My thinking as well. Michael was simply putting Fredo to the test.
Sonny, you could certainly infer that Michael was putting Fredo to the test in Havana, especially if you put weight on those suspicious looks Michael gives him. But it still leaves unanswered: If Michael did have any suspicions about Fredo, or if he was putting him to the test, why did he tell him about Roth's plan to have him assassinated--
and his own plan to have Roth whacked--especially since he concluded by telling Fredo to do nothing?