test: refactor test framework to use a rootfs skeleton

Instead of having to create files, and copying them in place for each
test, we move all static test files to a skeleton rootfs.  This makes
it a lot easier to get an overview of how things and how they work.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

test/skel/etc/init.d/rcS

@@ -0,0 +1,5 @@
+#!/bin/sh
+
+echo "Starting up ..."
+poweroff -d 2
+