Fix the PR 181741 regression test.

[markj]

Summary

When the corresponding data loss bug is fixed and the snd and rcv buffer
sizes are the same (the default configuration), the test would hang
because it is sending a message larger than the snd buffer size, which
activates SB_STOP. Fix the problem by using a non-blocking socket and
by allowing short writes to the socket in this particular test.

Test Plan

Verified that the test still fails without D16515, and passes with it.