test: keep the h2c unix socket path within sun_path limits

[mcollina]

Summary

test/h2c-client.js › Connect to h2c server over a unix domain socket creates its socket under mkdtemp(os.tmpdir()). Under CITGM on macOS, TMPDIR points inside the CITGM work directory (/private/var/folders/…/T/<id>/npm_config_tmp), so the socket path exceeds the 104-byte sun_path limit.

Node.js 22 silently truncates such paths (so the test happens to pass), while Node.js 24 and 26 reject them with EINVAL — which is why citgm undici fails on macOS with Node.js 24/26 only.

This falls back to /tmp when the tmpdir()-based path would be too long.

Refs: nodejs/citgm#1185

Verification

• With a ~110-char TMPDIR, the old test fails and the new one passes on Node.js 22.22.1, 24.21.0 and 26.9.0.
• npx borp -p test/h2c-client.js: 12/12 pass.

The other citgm failures in that issue (Node.js 22 onread->IsFunction() abort) are handled by #5961.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.52%. Comparing base (2def924) to head (f025e3c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5978   +/-   ##
=======================================
  Coverage   91.52%   91.52%           
=======================================
  Files         113      113           
  Lines       43537    43537           
=======================================
+ Hits        39847    39849    +2     
+ Misses       3690     3688    -2     

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