Psalm UndefinedDocblockClass for Expectable in functions.php

[ndench]

After upgrading to 2.9.x we get psalm reporting a lot of UndefinedDockblockClass issues on our pest tests.
It appears to be caused by 4f3796e.

This has been raised in psalm a while back (vimeo/psalm#3695) and the response was

Traits are not types so you shouldn't use them as types.

Maybe we should define an ExpectableInterface and use that type instead?

[nunomaduro]

That's an issue with psalm. Not Pest.

[kielabokkie]

@nunomaduro Would you reconsider this if PHPStan agrees with Psalm that Traits should not be used as types?

https://phpstan.org/writing-php-code/phpdoc-types#classes-and-interfaces