test "update rt period and runtime": fix up runtime and period
test "update rt period and runtime": fix up runtime and period

Fix #2046

Previously, the test was failing with EINVAL during writing 500001 to `/sys/fs/cgroup/cpu,cpuacct/runc-cgroups-integration-test/test-cgroup/cpu.rt_runtime_us`, because `/sys/fs/cgroup/cpu,cpuacct/runc-cgroups-integration-test/cpu.rt_runtime_us` was initialized with 0.

The issue had not been caught in Ubuntu 18.04 CI because it doesn't support rt.

Tested on Ubuntu 20.04.

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

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