Tests: Strip untypical callback parameter characters from PHP files

Tests: Strip untypical callback parameter characters from PHP files

Only allow alphanumeric characters & underscores for callback parameters.
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.

This is a 1.x/2.x version of PR gh-4871.

The change doesn't require a release; it's meant at installations testing
the latest state of `1.12-stable` & `2.2-stable` branches.

This change also fixes testing on Travis & on Chrome/Firefox.

Closes gh-4875
Ref gh-4764
Ref gh-4871