test: Port all 8 find-turbo prysk tests to Rust (#12066)
## Summary
- Ports all 8 `find-turbo/` prysk tests to Rust, clearing out the last
major prysk subdirectory.
- 17 new test functions covering local turbo binary discovery across all
node_modules layouts: hoisted, nested, pnpm-linked (symlinked), Yarn PnP
unplugged (default, moved via .yarnrc.yml, moved via YARN_RC_FILENAME
env).
- Tests verify version-dependent behavior: old turbo (< 1.8.0) gets bare
args, new turbo (>= 1.8.0) gets `--skip-infer --single-package`.
- Hard-mode tests cover `--skip-infer` self-detection, `--cwd`
navigation, and repo root discovery from nested directories.
- Adds `setup_find_turbo`, `set_find_turbo_version`, and
`set_find_turbo_link` helpers.
After this PR, only `new-package.t` remains as prysk (requires `pnpm
install` to update lockfiles, which can't be done portably in Rust
tests).
## For reviewers
The find-turbo tests use the fixture's fake `bin/turbo` shell scripts
(which just `echo $@`) as stand-ins for a locally-installed turbo. The
shim discovers these, checks their version from `package.json`, and
delegates to them. The test then asserts on the echoed args to verify
correct flag passing.
