Make PHPUnit tests subscribe users to notifications

Problem/Motivation

After fixing #3225558: Take notify_subscriptions into account, some automated PHPUnit tests fails. This is because before this was fixed, email notifications about new content was sent to all users whether they subscribed to content or not.

However, by default, no user is subscribed to anything. After the module started to take subscriptions into account, the tests stopped sending out notification emails, this makes the assertions about the number of emails sent out, fail.

Steps to reproduce

Run Kernel tests. Observe that they fail.

Proposed resolution

Modify the setup() function for the test classes that test sending out notifications to make subscribing to the 'article' content type the default for new users that are created.