CentOS 7 is showing its age and we'd rather skip some tests on it than
find out why they are flaky.
Add internal/testutil package, and move the generalized version of
SkipOnCentOS7 from libcontainer/cgroups/devices to there.
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
Kir Kolyshkin