[3.14] gh-154366: Fix test_asyncio.test_sendfile timing out on DragonFly BSD (GH...

[3.14] gh-154366: Fix test_asyncio.test_sendfile timing out on DragonFly BSD (GH-154367) (GH-154372)

A 4 KiB receive buffer makes DragonFly defer every window update to the
delayed ACK timer, so each buffer worth of data costs 100 ms.  Use a
larger socket buffer there; it is still small enough to pause the
protocol long before all data is sent.
(cherry picked from commit a6e1b836ec5055cbc8be477ca0c2df0cc12b8cb5)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>