Folder names in tests/build/* could use an improvement.

[mpolitzer]

Folder names in tests/build/* could use an improvement.
As of now, they are: cmio images machine misc uarch uarch-riscv-arch-test.

• cmio stores machines used by integration tests (could be improved).
• images stores linux.bin and rootfs.ext2 (seems fine).
• machine stores the ISA verification binaries (rv64gc) (could be improved).
• misc stores test-machine-c-api and test-merkle-tree-hash (seems fine).
• uarch stores the ISA verification binaries for uarch (rv64i?) (could be improved).
• uarch-riscv-arch-test stores more ISA verification binaries but from a separate test suite (also rv64gc) (could be improved).