gh-107851: Fix spurious failures in fcntl eintr tests (#121556)

gh-107851: Fix spurious failures in fcntl eintr tests (#121556)

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.