AssertTypeSpecifyingExtensionHelper: rootExpr with unknown variable to avoid always-true false positives

[janedbal]

This should fix https://github.com/phpstan/phpstan-phpunit/issues/193.

[janedbal]

Optionally, we might do this hack only for those resolvers where false-positive is at risk (like the assertContains with Traversable).

Edit: done, it breaks stuff without that.

[ondrejmirtes]

Thank you!