test(tmachine): move tmachine support files under tests/tmachine #4725

Problem Statement

tmachine's Ansible playbooks, cloud-init seed, and Nix config (config.nix, images.nix, artifacts.nix) sit at the top of tests/, next to the test suites, while the tmachine crate lives in tests/tmachine/. Everything that belongs to tmachine should live under tests/tmachine/, so tests/ holds only tmachine, the suites, and shared support.

This is the first step in landing the conformance harness from #4629 incrementally. Part of #3954.

Proposed Design

tests/
├── suites/      unchanged
├── support/     unchanged
└── tmachine/
    ├── Cargo.toml, src/   the tmachine crate (unmoved)
    ├── ansible/
    ├── cloud-init/
    ├── config.nix
    ├── images.nix
    └── artifacts.nix

Move the files and update the paths that refer to them. No behavior changes.

Acceptance Criteria

• nix run .#tmachine and the build-artifacts* apps work unchanged.
• Editing Ansible or Nix config does not rebuild the tmachine binary.
• All existing tmachine CI lanes (conformance, feature-specific, driver-specific) pass.