Controller_Test_Case - ensure PHPUnit invariance

[lucatume]

This is a follow-up to the #2062 and the Event Tickets #2977 one.

This fixes an issue where, depending on the PHPUnit version (TEC uses any version between 6 and 10 depending on the project, Codeception and wp-browser version), the set-up and tear-down methods of the Controller_Test_Case would not run in consistent order. With this change the code is reasonably isoalated.

There are no existing or new tests in Common proper for this change, since it would require a meta PHPunit setup, but I've created test PRs on ET and TEC pulling this Common branch to make sure all works fine:

• ET PR
• TEC PR