savestate backup had delete (it's asynchronous) vs move race condition

BizHawk.Client.EmuHawk/MainForm.cs

@@ -3804,6 +3804,11 @@ namespace BizHawk.Client.EmuHawk
 					backupFile.Delete();
 				}
 
+				//deletes are asynchronous, need to wait for it to be gone
+				//TODO - make a common utility method to replace a backup including this whole process
+				while (backupFile.Exists)
+					Thread.Sleep(10);
+
 				File.Move(path, backup);
 			}