test: use common.PIPE in fs.cp socket tests

[maksim-romanov]

Both tests created the socket at an absolute path under tmpdir/copy_%1/. On the x86_64-darwin and aarch64-darwin shared-library jobs that path is 106 bytes, over the 104-byte sun_path limit on macOS, so listen() fails with EINVAL and cpSync() then sees ENOENT instead of ERR_FS_CP_SOCKET.

Both tests now use common.PIPE, which is relative to the cwd, as the other pipe tests do.

I reproduced both failures locally by running the tests from a checkout with a 132-character path; they pass with this change (20 runs each, from both the long path and a regular checkout).

test-fs-cp-async-special-files-in-tree hits the same limit but only skips its socket case, and common.PIPE doesn't fit there since the socket has to live inside the copied tree, so I've left it for a separate change.

Fixes: #66324

Assisted-by: Claude Code

[github-actions]

Welcome to Node.js, and thank you for your first contribution!

Before review, please take a moment to read:

• the guide for first-time contributors
• the contribution and automation policies
• the pull request guide
• the AI use policy
• the Code of Conduct

Please make sure every commit is signed off. For a first pull request, GitHub Actions require collaborator approval and Jenkins CI must be started by a collaborator or triager, so an initial wait is normal.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/77975/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.36%. Comparing base (d35c89e) to head (1f56017).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66329      +/-   ##
==========================================
- Coverage   90.36%   90.36%   -0.01%     
==========================================
  Files         790      790              
  Lines      274657   274657              
  Branches    52587    52592       +5     
==========================================
- Hits       248207   248181      -26     
- Misses      16935    16945      +10     
- Partials     9515     9531      +16     

see 32 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[nodejs-github-bot]

Landed in ae9c25a