Fixes bug #21226.
Linux 3.19 introduced a fix for a security vulnerability in user namespaces.
This fix introduced a new proc file called 'setgroups' and was backported to
many older kernels. However, some users run a kernel that is new enough to
support user namespaces yet old enough to not include the patch, so we must
skip the tests.
* tests/containers.scm: Skip all tests if /proc/self/setgroups does not exist.
