Fix phpstan test

This fixes the phpstan issue!

Hasn't changed the phpunit result - there is specific handling of a webform compatibility issue but it doesn't appear to be directly tested.

There needs to be a better way to pass that test than to force us to now track webform versions in the composer.json.

Worst case, disabling phpstan, but I think the best would be to add a phpstan.neon file to ignore this one.

Phpstan needs all dependencies that have code that affects the project at hand (as either regular or dev dependencies) in order to do its static analysis. Ignoring this would mean that if webform_tokens ever changes its arguments, for example, phpstan will pass even though this module is now broken-- which is one of the problems we are hoping to avoid with phpstan.

Although we do need to hard-code a Webform version in composer.json, I tried to make it as broad as possible (any version 6). Webform 6 was released in 2020 and there seems to be no 7.x branch yet, so I hope this will not impose a greater maintenance burden.