Why does Fortunato laugh while he is chained to the wall?

Fortunato is hoping against hope that he can somehow get Montresor to release him. His laughter is forced. He does not see anything funny about what has happened to him. It is horrible. But he pretends to believe that he has been the victim of a practical joke and that he is sure Montresor has no intention of leaving him there to die. This would give Montresor an excuse for releasing him if Montresor had a change of heart. Fortunato makes his strategy clear when he says:

But is it not getting late? Will not they be awaiting us at the palazzo, the Lady Fortunato and the rest? Let us be gone.”

He wants Montresor to think that his wife and many other people are expecting him at his palazzo and that there will be relatives, friends, and servants out looking for him if he doesn't show up soon. (This is why he suggests that it is getting late.) He also says, "awaiting us" rather than "awaiting me." This is to suggest several things, mainly that a lot of people have seen them together walking in the direction of Fortunato's palazzo, and therefore there will be suspicion directed against Montresor if Fortunato disappears. But "awaiting us" also is intended to suggest that they are very good friends and that Montresor is always a welcome guest at Fortunato's home. This last is probably not true. Among the many "injuries" Fortunato has inflicted on Montresor are probably social snubs. Montresor has not been invited to important social functions at Fortunato's home, as he knows from reading the newspapers; He is treated more as an acquaintance than a friend of the family. Montresor is an outsider. Fortunato is hoping to frighten Montresor into releasing him and offering him a plausible explanation for doing so. But Montresor has previously ascertained that Fortunato has no "engagement." When Fortunato shows his eagerness to go to Montresor's palazzo to sample the purely fictitiouss Amontillado, Montresor suggests for the second time that he believes Fortunato is expected at home or somewhere else.

“My friend, no; I will not impose upon your good nature. I perceive you have an engagement. Luchesi—”

“I have no engagement;—come.”

That is what Montresor wanted to hear. He wants to leave a cold trail. No one is expecting Fortunato at home. He is out carousing and may not return home all night. And Montresor knows that releasing Fortunato now would be fatal. Fortunato might part on friendly terms, but he would be so outraged at being victimized by this "jest" that he would probably have Montresor murdered. 

Fortunato would like to believe that this is an "excellent jest," but he is about ninety-nine percent certain that it is not a jest. His hope that it might be a jest is what gives him the idea of pretending to think it is a jest. Nevertheless he has to laugh as if he is sure that it must be a practical joke, or "jest."