WCAG SCR31: test implementation and testcases for the test

Hi all!

Finally some code, but it is just an initial version that is broken right now.

This is re: #115 

A few questions regarding the test `scriptFocusIndicatorVisible.js` itself:

* does the CSS need to be reseted after my removing it in the first place? Tried searching for similar cases in the source, but there are a lot f tests. So I'd rather just ask it before spending time investigating all the tests ;-)

* is it specified how much the visual difference between the backgrounds / borders etc have to be? Right now the test checks for a difference, what if optically it is not noticeable?

* have I added all references from other files correctly or did I miss something?

Then about the tests of the test:

* I am not sure if I understand the test procedure done in `composite.js` correctly. Is the way the current test fails because of a bug in the test, or in the tests of the test?

Thanks :-)