Unit test core functionality directly

[alistaire47]

Per #110 (comment), ideally we want to test core functionality (mostly in _benchmark.py) directly, instead of just via implemented and example benchmarks. Some non-exclusive options:

1. Make more example benchmarks accessible from {arrowbench}
2. Put necessary R and JSON in testing resources
3. Mock out parts of classes in tests so smaller blocks of code can be tested