Send the Connection: close header the multipart test comment describes

expect_split_multipart_ok() carries a comment saying the request sends
"Connection: close" so the response drain ends as soon as the server has
answered, but the header itself never made it into the request, so both
callers kept idling until the read timeout instead.

Add the header. EpilogueSplitAcrossReadsIsIgnored and
InitialBoundarySplitAfterLongPreamble each drop from about 3.1s to about
0.11s.

test/test.cc

@@ -15474,6 +15474,7 @@ static void expect_split_multipart_ok(const std::string &body1,
   const std::string head =
       "POST /post HTTP/1.1\r\n"
       "Content-Type: multipart/form-data; boundary=zzzz\r\n"
+      "Connection: close\r\n"
       "Content-Length: " +
       std::to_string(body1.size() + body2.size()) + "\r\n\r\n";