This aligns the Node.js server with the previous PHP one in sending `mock.php`
as a callback if there's no `callback` parameter in the query string which is
triggered by a recently added test. This prevents the request crashing on that
Node.js server and printing a JS error:
```
TypeError: Cannot read property '1' of null
```

Closes jquerygh-4764
Ref jquerygh-4754

(cherry picked from commit df6858d)

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 jquerygh-4764

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

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

(cherry picked from a702746)

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 jquerygh-4871

Ref jquerygh-4764
Ref jquerygh-4871

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 jquerygh-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.

Ref jquerygh-4764
Ref jquerygh-4871

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 jquerygh-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.

Ref jquerygh-4764
Ref jquerygh-4871

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

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 jquerygh-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 jquerygh-4875
Ref jquerygh-4764
Ref jquerygh-4871

(cherry picked from acb7c49)