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

gh-141473: Speed up subprocess test_communicate_timeout_large_input long tail (#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.