[DNM] integration test for #3707

[kolyshkin]

This is an integration test for the fix in #3707

[kolyshkin]

I will now retry the same test with the fix removed

[kolyshkin]

OK, I have tested that the added case fails without the fix (#3707) and passes with it, as it should.

not ok 69 runc exec --user vs /dev/null ownership
# (in test file tests/integration/exec.bats, line 144)
#   `[ "$UG" = "0:0" ]' failed
# runc spec (status=0):
# 
# runc run -d --console-socket /tmp/bats-run-18871/runc.Cuk9gE/tty/sock test_busybox (status=0):
# 
# crw-rw-rw- 1 root root 1, 3 Feb  3 00:00 /dev/null
# uid=1000 gid=1000
# crw-rw-rw- 1 runneradmin root 1, 3 Feb  3 00:00 /dev/null