mysite.localhost works fine for very simple development, but can easily fall down with containers, VMs, or multi-machine development. Technically, you can point it to a non-loopback IP, but I wouldn't do that. .localdomain is used some places as well, but I don't know how reliable it is.
.test is a bit more flexible, and what I use now. I'd prefer if .dev was reserved as well, but that ship has sailed.
