resourceloader: Implement "skip function" feature

A module can be registered with a skip function. Such function,
if provided, will be invoked by the client when a module is
queued for loading. If the function returns true, the client will
bypass any further loading action and mark the module as 'ready'.

This can be used to implement a feature test for a module
providing a shim or polyfill.

* Change visibility of method ResourceLoader::filter to public.

So that it can be invoked by ResourceLoaderStartupModule.

* Add option to suppress the cache key report in ResourceLoader::filter.

We usually only call the minifier once on an entire request
reponse (because it's all concatenated javascript or embedded
javascript in various different closures, still valid as one
large script) and only add a little bottom line for the cache
key. When embedding the skip function we have to run the minifier
on them separately as they're output as strings (not actual
functions). These strings are typically quite small and blowing
up the response with loads of cache keys is not desirable in
production.

* Add method to clear the static cache of ResourceLoader::inDebugMode.

Global static state is evil but, as long as we have it, we at
least need to clear it after switching contexts in the test suite.

Also:
* Remove obsolete setting of 'debug=true' in the FauxRequest in
  ResourceLoaderTestCase. It already sets global wgResourceLoaderDebug
  in the setUp() method.

Bug: 66390
Change-Id: I87a0ea888d791ad39f114380c42e2daeca470961