Bug 991442 - Add coverage threshold to test-agent

https://bugzilla.mozilla.org/show_bug.cgi?id=991442
> In the aspect of build system, we should reduce the Makefile implementation and move our logic to Javascript (both xpcshell and node.js)

I'm ok doing the same thing in xpcshell/node js; I was just suggesting to do this in the same Makefile goal (that can call an external code, I don't mind), as this seems to be the same issue: give a gaia-specific list of files gaia-agnostic to test-agent :)