[TASK] Configure PHPUnit to be more strict

[oliverklee]

This change brings the PHPUnit configuration in line with what we have at Emogrifier (except for the difference in the XSD version number).

[coveralls]

coverage: 38.622%. remained the same
when pulling 9789866 on task/phpunit-configuration
into a5a1f1a on main.

[JakeQZ]

Changes seem mostly fine, but could we please keep the original indentation style. This would make the diff cleaner, and avoid attributes being idented by 9 spaces.

Also, do we need bootstrap="vendor/autoload.php". It seems to work without it, and must presumably be using that by default, otherwise the classes to be tested wouldn't load. Explicitly stating it creates a hard-coded value that makes assumptions about the behaviour of a third-party package (Composer itself). If PHPUnit effectively deals with the location of Composer's autoload file, then we should just allow it to do so.

[oliverklee]

Done and repushed.

[JakeQZ]

I see clearly now we already had cacheResult="false" in this project :)