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

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

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.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>