internal/runtime/cgroup: simplify escapePath in test

internal/runtime/cgroup: simplify escapePath in test

Don't work on rune, kernel does not use utf-8 here.
Can be verified like this:

  # mkdir "$(echo -e "\xff\x20")"
  # mount -t tmpfs tmpfs "$(echo -e "\xff\x20")"
  # tail -n 1 /proc/self/mountinfo | xxd
  00000000: 3133 3334 2031 3030 2030 3a31 3632 202f  1334 100 0:162 /
  00000010: 202f 726f 6f74 2fff 5c30 3430 2072 772c   /root/.\040 rw,
  00000020: 7265 6c61 7469 6d65 2073 6861 7265 643a  relatime shared:
  00000030: 3433 3520 2d20 746d 7066 7320 746d 7066  435 - tmpfs tmpf
  00000040: 7320 7277 0a                             s rw.

Change-Id: I7468b56eb26f14bc809f8f7580535e6562795c62
Reviewed-on: https://go-review.googlesource.com/c/go/+/723300
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>