Add tips on testing systems to guide

I've been going through the specs guide as I'm consider porting my WIP game https://akigi.com to use `specs` instead of my own ECS.

The guide has been excellent and has set me up to be able to port with _relative_ ease.

One thing that feels missing is advice and guidance around writing tests for systems.

It would be awesome if there was a dedicated chapter on the subject - with a few examples of good system tests.

I'm sure that I'll be able to dig around the API and figure out my own best practices for writing my tests since `specs` has such an intuitive API - but it'd be nice if instead of spending somewhere between 30-120 mins thinking through that the guide just gave me guidance from the start like it it did for everything else!

Thanks for the consideration - cheers!