tests/int: fix "update rt period and runtime" for rootless

Since commit f09a3e1, the value passed on to read starts with
a slash, resulting in the first element of the array to be empty.

As a result, the test tries to write to the top-level cgroup, which
fails when rootless:

> # Writing 1000000 to /sys/fs/cgroup/cpu,cpuacct//cpu.rt_period_us
> # /tmp/bats-run-106184/bats.115768.src: line 548: /sys/fs/cgroup/cpu,cpuacct//cpu.rt_period_us: Permission denied

To fix, remove the leading slash.

An alternative fix would be to do "for ((i = 1;" instead of "i = 0", but
that seems less readable.

Fixes: f09a3e1
Signed-off-by: Kir Kolyshkin <[email protected]>

tests/integration/update.bats

@@ -537,7 +537,7 @@ 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 <<<"$REL_CGROUPS_PATH"
+	IFS='/' read -r -a dirs <<<"${REL_CGROUPS_PATH#/}"
 	for ((i = 0; i < ${#dirs[@]}; i++)); do
 		local target="$CGROUP_CPU_BASE_PATH"
 		for ((j = 0; j <= i; j++)); do