Have the CPU benchmark abort if the user leaves the lobby.

An edgecase, but it's been annoying me: The CPU benchmark runs to completion even if the user returns to the main menu (which, admittedly, can only happen if you're in a LAN game).

While I'd dearly like to just exterminate this ridiculous benchmark entirely, this should go some way to making it less irritating.

More generally, we should perhaps consider replacing it with something that gives a more accurate picture of how well a machine will cope in-game. Testing the performance of Lua arithmetic operators isn't a sensible way to do this: this is not the nature of the majority of the game's load.
Realistically, the best way to produce a rating would be just to read system information (whatever windows' equivalent of `lspci`, `/proc/cpuinfo` and `/proc/meminfo` are) and make a decision based on that. As-is, the benchmark often spits out slightly stupid numbers that aren't really representative of how well a machine will perform in-game, and causes many seconds of UI unusability while it's thinking about it...