Specify file paths instead of function and class names

[johnbillion]

Instead of specifying function and class names, the test case should instead specify a list of file paths which are then statically scanned for functions and classes, and tested.

This can be done using phpDocumentor/Reflection. The main problem this causes is that the dataProvider for the test needs to return a list of functions and class methods, which can get extremely long and - for a project such as WordPress - actually takes a considerable amount of time to generate.

I've done this previously as a patch on https://core.trac.wordpress.org/ticket/35065 but it needs some more work.