tests/int: fix runc exec --preserve-fds
tests/int: fix runc exec --preserve-fds

This test started to fail, as it expected the output from
`id` to be

	uid=1000 gid=1000 groups=100(users),65534(nogroup)

while the actual output is now

	uid=1000 gid=1000 groups=100(users),65534(nobody)

Apparently, busybox image changed group name.

As we're only interested in ids, not names, and to fix the test,
let's use `id -G`.

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

opencontainers/runc,xiaochenshen/runc,kolyshkin/runc,estesp/runc,rhatdan/runc,rhatdan/runc,opencontainers/runc,mikebrow/runc,dqminh/runc,estesp/runc,kolyshkin/runc,rhatdan/runc,estesp/runc,mikebrow/runc,dqminh/runc,dqminh/runc,xiaochenshen/runc,dqminh/runc,opencontainers/runc,opencontainers/runc,mikebrow/runc,kolyshkin/runc,xiaochenshen/runc,kolyshkin/runc,rhatdan/runc