tool test: prepare test python-3.14.8; cl/cltest: TestFromDir support select single dir

[xushiwei]

No description provided.

[fennoai]

Review summary

Small, well-scoped change that adds single-directory selection to cltest.TestFromDir and reorganizes the C test fixtures (./_testc → ./_testc/clang-c-22.1.8), plus a staged python-3.14.8 fixture. The core logic is correct:

• Single-level relDir (e.g. ./_testc, ./_testcpp) keeps the original os.ReadDir behavior, so existing callers are unaffected.
• A multi-level relDir is correctly reconstructed: fs.NewDirInfo(fname) yields an entry whose IsDir() is true, passing the fi.IsDir() filter, and pkgDir = dir + "/" + name round-trips to the original path. Verified against qiniu/x v1.18.3.

Security / performance: no issues. The CI push trigger narrowing in .github/workflows/llgo.yml is benign — PRs still run on pull_request: [ "**" ], so merge-gating coverage is unchanged. The new go.sum hashes for goplus/lib v0.5.11 and qiniu/x v1.18.3 match the checksum database.

Inline findings are minor/non-blocking (naming + stale doc comments). A couple of low-priority notes that don't map cleanly to a changed line:

• Mixed path abstractions (cltest.go): the function builds dir with path.Join (slash-based) but the new branch uses filepath.Split + string(os.PathSeparator). Harmless on Linux/macOS, but inconsistent with the surrounding slash-based style.
• Unreferenced fixture: tool/_testc/python-3.14.8/ adds a module + vendored Python headers, but no test references ./_testc/python-3.14.8 yet. Looks intentionally staged for a follow-up (commit messages say "prepare") — just confirming it is not expected to be exercised in this PR.