Refactor `uarch-riscv-tests.lua`. Similar to commit 4c399594e1f3f29173b334353193b2ab3f833d72

[mpolitzer]

• Cleanup lua code, similar to commit: https://github.com/cartesi/machine-emulator/commit/4c399594e1f3f29173b334353193b2ab3f833d72
• Add support for parallel execution