Added section about choosing test variables

I do think that this is something to consider. Haven't changed anything about this at the moment as I do not think it fits too well in the current three sections as they all seem to be once off data, but maybe something like server response ids if you keep hitting a server? Don't want to assume that those are sequential.

I will try think of a good way to word this, else if  you come up with a better example let me know.