autotests: Rewrite testScreenEdges as an integration test

Due to the screen edges test not being an integration test, it's very
hard to change output related code in libkwin. screens.cpp needs to have
a few ifdefs to successfully compile.

This change rewrites the screen edges test as an integration test in
order to allow us using other components of kwin in screens.cpp and
screenedge.cpp without ifdef guards.

It's not a one-to-one port.