Write tests for interface callbacks

[Cloudef]

Make sure nothing is called on wlc_init.

Make sure they are called on correct order, eg. view.request.geometry before view.created is not valid.

[xeechou]

Is there anything you can debug when wlc_init returns false? I just updated my wlc-git to 0.0.5 on Archlinux and my wlc_init returned false all the time. It used to work though

[Cloudef]

wlc_init implementation can't really return false. It will always kill the process with error message if something goes wrong.