Restricted functions used in callbacks

While debugging the AbstractFunctionRestrictionsSniff class, I realized that any restricted functions used as callbacks, i.e. within add_action(), call_user_func(), array_map() are completely ignored.

That makes this kind of sniff extremely easy to bypass and while this is not so much an issue for people who elect to use the WPCS, this will be an issue for the Theme Review Theme Check to sniffs project as in that case, theme authors won't have a choice and bypassing checks that way is something we'll need to guard against.

I'm investigating how we can solve this. /cc @grappler

Start by reading the AbstractFunctionRestrictionsSniff class and tracing how it handles direct restricted-function usage. Reproduce the gap with callbacks passed to add_action(), call_user_func(), and array_map(). Done means restricted functions used in those callback contexts are no longer ignored and the relevant sniff behavior is covered by tests, though no test file is named in the issue.