Add testcase for #1857

bin/varnishtest/tests/r01857.vtc

+varnishtest "Check session herding"
+
+server s1 {
+	rxreq
+	txresp -hdr "Connection: close" -body "012345\n"
+} -start
+
+varnish v1 -vcl+backend { } -start
+
+client c1 {
+	txreq -url "/"
+	rxresp
+	expect resp.status == 200
+	expect resp.http.X-Varnish == "1001"
+	delay 1
+	txreq -url "/"
+	rxresp
+	expect resp.status == 200
+	expect resp.http.X-Varnish == "1003 1002"
+} -run
+
+# Give varnish a chance to update stats
+delay .1
+
+varnish v1 -expect sess_herd == 1
+varnish v1 -expect sess_conn == 1
+varnish v1 -expect cache_hit == 1
+varnish v1 -expect cache_miss == 1
+varnish v1 -expect client_req == 2
+varnish v1 -expect s_sess == 1
+varnish v1 -expect s_req == 2
+varnish v1 -expect s_fetch == 1