Compile tests with riscof run

[Scott-Guest]

• Implement riscof_kriscv.py enough so that we can compile (but not execute) tests with riscof run
• Remove the compilation logic from test_conformance.py, and instead call riscof run in make test-integration

Additionally,

• Move the tests/riscof folder to src/tests/integration/riscof
• Delete the sail_cSim reference plugin template
  • Nothing we are doing actually runs a reference plugin yet, we may not end up using sail_cSim, and the template would have required significant modification anyways, so no point leaving this checked-in

A few files get shuffled around, so I'd recommend reviewing this commit-by-commit.

[sskeirik]

This looks good overall to me. I only did a quick perusal since I was busy today. I left a few possible nits to consider.