cgroup: Add config file to cgroup selftest suite

Most of the test suites in tools/testing/selftests contain a config file
that specifies which kernel config options need to be present in order for
the test suite to be able to run and perform meaningful validation. There
is no config file for the tools/testing/selftests/cgroup test suite, so
this patch adds one.

Signed-off-by: David Vernet <void@manifault.com>
Signed-off-by: Tejun Heo <tj@kernel.org>