Rework null adapter into mock adapter

[aarongreig]

This also comes with a new accompanying helper header which allows tests to override entry points and create their own reference counted dummy handles.

[aarongreig]

@pbalcer This is a working (insofar as I have one of the sycl unittest suites passing with it) prototype of the mock adapter + header approach. The interesting stuff is in scripts/core/loader.yml where there's a new loader entry point to force load the mock adapter (and the implementation for that in source/loader/ur_lib.cpp) and in source/mock where the mock interface header lives. If the overall approach looks ok to you I'll come back to polish and add docs/testing

[pbalcer]

Looks good. Much cleaner than trying to shoehorn this functionality into a layer :)

[aarongreig]

this is now complete with docs and testing, it depends on #1720 but since that's a NFC I'll be able to merge it pretty soon

[pbalcer]

lgtm

[isaacault]

Pending the changes added from reviews in #1720, this LGTM.