[3.13] gh-107851: Fix spurious failures in fcntl eintr tests (GH-121556) (#121585)

On heavily loaded machines, the subprocess may finish its sleep before
the parent process manages to synchronize with it via a failed lock.

This leads to errors like:

  Exception: failed to sync child in 300.3 sec

Use pipes instead to mutually synchronize between parent and child.
(cherry picked from commit af9f6de6ea930b607f948f2c91a87fe4ca9d64db)

Co-authored-by: Sam Gross <colesbury@gmail.com>