Backend send_timeout coverage

Despite v1l code being used for both client responses and backend
requests, the send timeouts only kicks in on the client side. The
backend code does not enter the send loop subject to the timeout
for lack of niov at V1L_Open() time.

Refs 43bbe97b

bin/varnishtest/tests/s00011.vtc

+varnishtest "backend send timeouts"
+
+server s1 {
+	rxreq
+	expect req.method == POST
+	expect req.body == helloworld
+	txresp
+} -start
+
+varnish v1 -vcl+backend "" -start
+
+# Despite shared code, these parameters are only for client responses.
+# See 43bbe97b38a86d706c0d9f4838bbfbd305342775.
+
+varnish v1 -cliok "param.set idle_send_timeout 1"
+varnish v1 -cliok "param.set send_timeout 1"
+
+client c1 {
+	txreq -method POST -hdr "Content-Length: 10"
+	send hello
+	delay 2
+	send world
+	rxresp
+	expect resp.status == 200
+} -run