security_Minijail0: add test for entering user namespace

First check the uid/gid inside minijail, and create a temporary file.
Then check the uid/gid of that file outside minijail. So that we are
sure the uid/gid mappings are setup correctly.

BUG=chromium:517387
TEST=Passes on samus
CQ-DEPEND=CL:291200

Change-Id: Ib32d491672b0eb608f9830288fd47b1fc67f38b6
Reviewed-on: https://chromium-review.googlesource.com/291560
Commit-Ready: Yu-hsi Chiang <yuhsi@google.com>
Tested-by: Yu-hsi Chiang <yuhsi@google.com>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
8 files changed