After rereading Rambam a couple of times, I think I disagree with your interpretation of his writing. The part you cut out reads:
"Can there be a greater stumbling block than Christianity? All the prophets spoke of Mashiach as the redeemer of Israel and their savior who would gather their dispersed and strengthen their observance of the mitzvot. In contrast, Christianity caused the Jews to be slain by the sword, their remnants to be scattered and humbled, the Torah to be altered, and the majority of the world to err and serve a god other than the Lord."
It isn't that Christianity is itself a test for Jews, it's that the effects of Christianity caused tremendous difficulties for Jews.
The second paragraph (which you quoted) then goes on to assure the reader that, for lack of a better phrase, "everything is in the hand of heaven" - even the negative things which he mentioned in the previous paragraph are all part of the ultimate plan and leading to the ultimate salvation.
I apologize if I'm splitting hairs (and I did try to fit this in a comment and failed), but I don't think Rambam is saying that Christianity is a test for Jews.