Print Preview: Move policy tests

Moving policy tests to new model_settings_policy_test.js, similar to
what was done with the availability tests, since almost all the logic
tested is in model.js.

Also adding 2 small tests verifying that the corresponding settings
sections disable their controls when the setting is set by policy.

Change-Id: Ife0563e517a002ddd29a3fa62031fcd7ec47136e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1501948
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Esmael El-Moslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637874}
5 files changed