tests: shell: add cgroupv2 socket match test case

tests: shell: add cgroupv2 socket match test case

This is a test case for nft_socket cgroupv2 matching, including
support for matching inside a cgroupv2 mount space added in kernel
commit 7f3287db6543 ("netfilter: nft_socket: make cgroupsv2 matching work with namespaces").

Test is thus run twice, once in the initial namespace and once with
a changed cgroupv2 root.

In case we can't create a cgroup or the 2nd half (unshared re-run)
fails, indicate SKIP.

Signed-off-by: Florian Westphal <fw@strlen.de>