tests: improve rootless_cg testing
tests: improve rootless_cg testing

This ensures that we don't hard-code the set of cgroups on the host, as
well as making the permissions granted by rootless.sh much more
restrictive (to improve the scope of testing).

Signed-off-by: Aleksa Sarai <a8eb12d32d6ec07cdf737175ec7d4a0f1b4e1ee7@suse.de>

estesp/runc,wking/runc,justincormack/runc,rajasec/runc,laijs/runc,dqminh/runc,kolyshkin/runc,estesp/runc,adrianreber/runc,crosbymichael/runc,dqminh/runc,mYmNeo/runc,cyphar/runc,opencontainers/runc,mYmNeo/runc,stefanberger/runc,adrianreber/runc,justincormack/runc,xiaochenshen/runc,mheon/runc,sunyuan3/runc,rhatdan/runc,opencontainers/runc,kolyshkin/runc,sunyuan3/runc,mikebrow/runc,mikebrow/runc,opencontainers/runc,mikebrow/runc,cyphar/runc,xiaochenshen/runc,estesp/runc,rhatdan/runc,dqminh/runc,kolyshkin/runc,cyphar/runc,mheon/runc,laijs/runc,wking/runc,rhatdan/runc,crosbymichael/runc,rajasec/runc,laijs/runc,mYmNeo/runc,rhatdan/runc,wking/runc,opencontainers/runc,dqminh/runc,justincormack/runc,rajasec/runc,crosbymichael/runc,adrianreber/runc,kolyshkin/runc,stefanberger/runc,mheon/runc,sunyuan3/runc,stefanberger/runc,xiaochenshen/runc