Expand loader validation

[pbalcer]

Currently the loader validation has gaps in areas inherited from the original code base, which may result in bugs in its less frequently utilized parts, such as #598.

This task is to analyze coverage of the existing tests (and ideally we'd set up codecov to automate this), and expand the scope of testing to address the found gaps. At the very least, we need to make sure that all existing permutations of functions from loader templates are covered and functional. Instead of trying to call every single function, we need to make sure that tests call every function that has some specific way in which it is generated.
This will likely require us to expand the null adapter to implement more functions.

[kbenzie]

Would this be a good candiate for an initial pass at testing with fuzzing #566 ?

[pbalcer]

Yes, that's what @PatKamin is effectively thinking of doing. But, instead of expanding the null adapter, we might leverage the level-zero adapter to avoid extra work.