Add user and group for tinyproxy

This CL will enable running a proxy server in a tast test.

BUG=chromium:1134511
TEST=none

Change-Id: I27a4f387ee9a23c7560d0703139c8852ca50eb66
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/eclass-overlay/+/2453549
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Tested-by: Andreea-Elena Costinas <acostinas@google.com>
Commit-Queue: Andreea-Elena Costinas <acostinas@google.com>