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

This fixes the following warning, and implements a suggestion:

> In update.bats line 426:
>     IFS='/' read -r -a dirs <<< $(echo ${CGROUP_CPU} | sed -e s@^${CGROUP_CPU_BASE_PATH}/@@)
>                                 ^-- SC2046: Quote this to prevent word splitting.
>                                   ^-- SC2001: See if you can use ${variable//search/replace} instead.

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

tests/integration/update.bats

@@ -423,7 +423,7 @@ EOF
     root_period=$(cat "${CGROUP_CPU_BASE_PATH}/cpu.rt_period_us")
     root_runtime=$(cat "${CGROUP_CPU_BASE_PATH}/cpu.rt_runtime_us")
     # the following IFS magic sets dirs=("runc-cgroups-integration-test" "test-cgroup")
-    IFS='/' read -r -a dirs <<< $(echo ${CGROUP_CPU} | sed -e s@^${CGROUP_CPU_BASE_PATH}/@@)
+    IFS='/' read -r -a dirs <<< "${CGROUP_CPU//${CGROUP_CPU_BASE_PATH}/}"
     for (( i = 0; i < ${#dirs[@]}; i++ )); do
         local target="$CGROUP_CPU_BASE_PATH"
         for (( j = 0; j <= i; j++ )); do