Fix copy-paste bug in command-line playtesting code

Found this because when I compiled with Clang, -Wself-assign-field was
enabled.

desktop_version/src/Game.cpp

@@ -5196,7 +5196,7 @@ void Game::customloadquick(std::string savfile)
 {
     if (cliplaytest) {
         savex = playx;
-        savey = savey;
+        savey = playy;
         saverx = playrx;
         savery = playry;
         savegc = playgc;