bpo-33353: test_asyncio use set_write_buffer_limits() (GH-7200)


Use transport.set_write_buffer_limits() in sendfile tests of
test_asyncio to make sure that the protocol is paused after sending
4 KiB. Previously,
test_sendfile_fallback_close_peer_in_the_middle_of_receiving() failed
on FreeBSD if the DATA was smaller than the default limit of 64 KiB.
(cherry picked from commit 9551f7719213243fd96c4f284079243773c26b3c)

Co-authored-by: Victor Stinner <vstinner@redhat.com>
diff --git a/Lib/asyncio/streams.py b/Lib/asyncio/streams.py
index 9a53ee4..d6531f8 100644
--- a/Lib/asyncio/streams.py
+++ b/Lib/asyncio/streams.py
@@ -16,7 +16,7 @@
 from .tasks import sleep
 
 
-_DEFAULT_LIMIT = 2 ** 16
+_DEFAULT_LIMIT = 2 ** 16  # 64 KiB
 
 
 class IncompleteReadError(EOFError):