tests/int/update.bats: fix a shellcheck warning
tests/int/update.bats: fix a shellcheck warning

Fixes the following warning:

> In update.bats line 422:
>     local root_period=$(cat "${CGROUP_CPU_BASE_PATH}/cpu.rt_period_us")
>           ^---------^ SC2155: Declare and assign separately to avoid masking return values.
>
>
> In update.bats line 423:
>     local root_runtime=$(cat "${CGROUP_CPU_BASE_PATH}/cpu.rt_runtime_us")
>           ^----------^ SC2155: Declare and assign separately to avoid masking return values.

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

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