Move Ceedling vendor files to common location

This is a proof of concept for moving the Ceedling build files to a common directory.

@kytrinyx, is there support for adding common exercise-agnostic files?

* I recall that this same idea of a common folder was proposed for the [xCpp](https://github.com/exercism/xcpp) track, but a different approach was taken.
* The [xCpp](https://github.com/exercism/xcpp) track requires that users install Boost on their system, so exercism does not need to provide the unit test framework for that language.