tests/rootless.sh: fixup for "update rt" test
tests/rootless.sh: fixup for "update rt" test

Without this, the test case fails with

> Writing 1000000 to /sys/fs/cgroup/cpu,cpuacct/runc-cgroups-integration-test/cpu.rt_period_us
> /tmp/bats-run-106836/bats.116418.src: line 548: /sys/fs/cgroup/cpu,cpuacct/runc-cgroups-integration-test/cpu.rt_period_us: Permission denied

Since we do not currently have a setup to test this, this went
unnoticed (can be seen in RHEL8 though).

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>
Signed-off-by: Adrian Reber <9f82b3db7475afbb412180fe9fab7c69d6ece340@redhat.com>

kolyshkin/runc,dqminh/runc,opencontainers/runc,kolyshkin/runc,opencontainers/runc,kolyshkin/runc,kolyshkin/runc,rhatdan/runc,dqminh/runc,rhatdan/runc,opencontainers/runc,dqminh/runc,rhatdan/runc,dqminh/runc,opencontainers/runc,rhatdan/runc