os/user: user random name for the test user account

os/user: user random name for the test user account

TestImpersonated and TestGroupIdsTestUser are flaky due to sporadic
failures when creating the test user account when running the tests
from different processes at the same time.

This flakiness can be fixed by using a random name for the test user
account.

Fixes #73523
Fixes #74727
Fixes #74728
Fixes #74729
Fixes #74745
Fixes #74751

Cq-Include-Trybots: luci.golang.try:gotip-windows-amd64-longtest
Change-Id: Ib2283a888437420502b1c11d876c975f5af4bc03
Reviewed-on: https://go-review.googlesource.com/c/go/+/690175
Auto-Submit: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Bypass: Dmitri Shuralyov <dmitshur@golang.org>