[chromeos] Simplify chromeos device policy tests

[chromeos] Simplify chromeos device policy tests

- Remove the hacks that were there to accommodate /root/.forget_usernames
- Make setUp() re-entrant. This is so that errors in a test's setup does
  not affect the next test. (DNS server and auth server start at a fixed
  port, so failure to clean them up would affect the next test)
- Work around crbug.com/139166

BUG=139166
TEST=chromeos_device_policy.py

R=bartfab@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=149012

[Nirnimesh, 2012-07-27 19:26:35]

This CL fixes the large scale errors with enterprise tests. A test's error or
failure should not affect the succeeding test.

[bartfab (slow), 2012-07-30 08:59:44]

LGTM with nits. Great work - I hope this will make our policy tests a lot more
reliable.

http://codereview.chromium.org/10830053/diff/6001/chrome/test/functional/chro...
File chrome/test/functional/chromeos_device_policy.py (right):

http://codereview.chromium.org/10830053/diff/6001/chrome/test/functional/chro...
chrome/test/functional/chromeos_device_policy.py:5: import logging
Nit: "logging" is no longer needed by this file

http://codereview.chromium.org/10830053/diff/6001/chrome/test/functional/chro...
chrome/test/functional/chromeos_device_policy.py:15: # owner.
Nit: This comment is actually no longer true. Now that the TestServer (mock
DMServer) is used, it decides who the owner is. And the owner is hard-coded in
that server as user@example.com.

http://codereview.chromium.org/10830053/diff/6001/chrome/test/pyautolib/chrom...
File chrome/test/pyautolib/chromeos/suid_actions.py (right):

http://codereview.chromium.org/10830053/diff/6001/chrome/test/pyautolib/chrom...
chrome/test/pyautolib/chromeos/suid_actions.py:15: import logging
Nit: "logging" is no longer needed by this file