tests/int: fix wrt busybox's nogroup->nobody change

[kolyshkin]

Apparently, newer busybox image changed "nogroup" group name to "nobody",
breaking a few test cases that depended on the name.

Fix those tests by switching to numeric IDs.

1. 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.

2. tests/int/dev.bats: fixes for new busybox

These tests expect group name to be "nogroup", while recent busybox
changed that to "nobody".

Use numeric uids/gids to fix.

[kolyshkin]

@AkihiroSuda @mrunalp PTAL

[kolyshkin]

Found while looking at CI for #2580

[mrunalp]

@AkihiroSuda ptal

[kolyshkin]

Another failure caused by the same change:

not ok 28 runc run [redundant default /dev/tty]
# (in test file tests/integration/dev.bats, line 22)
#   `[[ "${lines[0]}" =~ "crw-rw-rw".+"1".+"nobody".+"nogroup".+"5,".+"0".+"/dev/tty" ]]' failed
....
# crw-rw-rw-    1 nobody   nobody      5,   0 Sep  9 01:06 /dev/tty

[chuanchang]

I also hit the same issue, it's nobody not nogroup.