[3.14] gh-141473: Speed up subprocess test_communicate_timeout_large_input long tail...

[3.14] gh-141473: Speed up subprocess test_communicate_timeout_large_input long tail (GH-149003) (#149004)

gh-141473: Speed up subprocess test_communicate_timeout_large_input long tail (GH-149003)

gh-141473: Speed up test_communicate_timeout_large_input

Replace the slow reader's 30s sleep with a parent-driven wake over a
loopback socket so post-timeout communicate() doesn't block waiting
for the child to wake on its own. Worst-case runtime: ~30s -> <1s.
(cherry picked from commit e1384cfd25b4fba5e0f8f3e6b536930e2e6cf5cf)

Co-authored-by: Gregory P. Smith <68491+gpshead@users.noreply.github.com>