[31628] made it possible to pass an `$autoload` param to `update_option()` that
applies when the option does not yet exist in the database. The current
changeset introduces parity for existing options: the `$autoload` setting
for existing options can be changed via the `$autoload` parameter. For internal
simplicity, `$autoload` is ignored for existing options when `$value` is not
also changed.
This changeset also moves `update_option()` tests into their own class.
Props dd32.
Fixes#26394.
git-svn-id: https://develop.svn.wordpress.org/trunk@31640 602fd350-edb4-49c9-b593-d223f7449a82