Add test for #1608

We don't return 413/414 (yet?) but we have a specific counter for this
now.  Whether this is enough it's something we need to finalise.

bin/varnishtest/tests/r01608.vtc

+varnishtest "Increment counter if http_req_size is exhausted"
+
+server s1 {
+} -start
+
+varnish v1 -arg "-p http_req_size=2048" -vcl+backend {
+} -start
+
+client c1 {
+	send "GET /"
+	send_n 2048 "A"
+	send " HTTP/1.1\r\n\r\n"
+	expect_close
+} -run
+
+varnish v1 -expect sc_rx_overflow == 1