SELinux denies access to some files in /sys, so we can't just trawl
through that asserting general truths. Instead, create a small known
tree.
Sadly neither ftw nor nftw takes user callback data, otherwise it would
be nice to assert that we visit all the expected nodes.
Bug: 19252748
Change-Id: Ib5309c38aaef53e6030281191a265a8d5a619044
