It turns out the times are not important; the hangup is the default size of the socket buffers on OS X and possibly BSD in general.  In my case, the send and receive buffers are 8192, which explains why the chunks written are so small.  I somewhat arbitrarily changed the sizes of the buffers in _test_send with:

rd.setsockopt(socket.SOL_SOCKET, socket.SO_RCVBUF, support.SOCK_MAX_SIZE // 3)
wr.setsockopt(socket.SOL_SOCKET, socket.SO_SNDBUF, support.SOCK_MAX_SIZE // 3)

The results were:

test_send (__main__.SocketEINTRTest) ... rd SO_RCVBUF default was 8192, wr SO_SNDBUF default was 8192
len(data) = 50331651
 sent = 5592405, written = 5592405
 chunk = 5592405, total read = 5592405
 chunk = 5592405, total read = 11184810
 chunk = 5592405, total read = 16777215
 sent = 16777215, written = 22369620
 chunk = 5592405, total read = 22369620
 chunk = 5592405, total read = 27962025
 chunk = 5592405, total read = 33554430
 chunk = 5592405, total read = 39146835
 sent = 22369620, written = 44739240
 chunk = 5592405, total read = 44739240
 sent = 5592411, written = 50331651
 chunk = 5592405, total read = 50331645
 chunk = 6, total read = 50331651
ok
test_sendall (__main__.SocketEINTRTest) ... rd SO_RCVBUF default was 8192, wr SO_SNDBUF default was 8192
len(data) = 50331651
 chunk = 5592405, total read = 5592405
 chunk = 5592405, total read = 11184810
 chunk = 5592405, total read = 16777215
 chunk = 5592405, total read = 22369620
 chunk = 5592405, total read = 27962025
 chunk = 5592405, total read = 33554430
 chunk = 5592405, total read = 39146835
 chunk = 5592405, total read = 44739240
 sent = None, written = 50331651
 chunk = 5592405, total read = 50331645
 chunk = 6, total read = 50331651
ok
test_sendmsg (__main__.SocketEINTRTest) ... rd SO_RCVBUF default was 8192, wr SO_SNDBUF default was 8192
len(data) = 50331651
 sent = 5592405, written = 5592405
 chunk = 5592405, total read = 5592405
 chunk = 5592405, total read = 11184810
 chunk = 5592405, total read = 16777215
 chunk = 5592405, total read = 22369620
 chunk = 5592405, total read = 27962025
 sent = 27962025, written = 33554430
 chunk = 5592405, total read = 33554430
 chunk = 5592405, total read = 39146835
 chunk = 5592405, total read = 44739240
 sent = 16777221, written = 50331651
 chunk = 5592405, total read = 50331645
 chunk = 6, total read = 50331651
ok

I dunno if a value that large will work in all environments, so the code to call setsockopt might need to be smarter.