Fix WebAssert since PHPStan can now check the number of arguments passed to methods

Problem/Motivation

In #3261266: Remove deprecated code from the testing framework (base classes, listeners, etc), we kept a layer to control that WebAssert methods do not get called with a wrong number of parameters, waiting for PHPStan for a better solution.

Now the solution is there: PHPStan level 1 rule; and we can get rid of that layer.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet