fix `libcontainer/integration/exec_test.go:1859:8: undefined: ioutil`
fix `libcontainer/integration/exec_test.go:1859:8: undefined: ioutil`

Fix 4d176544794c0a75f4dd58fe9e971361c66a4756

Signed-off-by: Akihiro Suda <10ddef0dea2a66ab193b07198faa879adcb213d8@hco.ntt.co.jp>

kolyshkin/runc,opencontainers/runc,kolyshkin/runc,kolyshkin/runc,opencontainers/runc,opencontainers/runc,kolyshkin/runc,opencontainers/runc