Configure TestTag to be allowed on a class.

[doppynl]

See https://github.com/DaveLiddament/php-language-extensions/issues/25

Perhaps it is best to wait with merging this one until an implementation in DaveLiddament/phpstan-php-language-extensions. I'm not sure yet how to prevent false errors on the class itself, is it should be possible to interact with itself. I'll also update the examples here when the implementation is done.