Tests: Strip untypical callback parameter characters from mock.php

Tests: Strip untypical callback parameter characters from mock.php

Only allow alphanumeric characters & underscores for callback parameters.
The change is done both for the PHP server as well as the Node.js-based version.
This is only test code so we're not fixing any security issue but it happens
often enough that the whole jQuery repository directory structure is deployed
onto the server with PHP enabled that it makes is easy to introduce security
issues if this cleanup is not done.

Ref gh-4764
Closes gh-4871