[SettingsSplit] Basic infrastructure for OS settings tests.

This CL just creates an infrastructure based on
cr_settings_browsertest.js and runs the existing test of the
settings-smb-shares-page element on the newly migrated file
chrome/browser/resources/settings/chromeos/os_downloads_page/smb_shares_page.js.

Bug: 966595
Change-Id: I5af3a28877a74474e3b715559e4a513d07c9119a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1628014
Commit-Queue: Jordy Greenblatt <jordynass@chromium.org>
Reviewed-by: Jordy Greenblatt <jordynass@chromium.org>
Reviewed-by: May Lippert <maybelle@chromium.org>
Reviewed-by: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664022}