add test r01755.vtc

src/tests/r01755.vtc

+varnishtest "The (struct backend).n_conn counter is never decremented"
+
+server s1 {
+	rxreq
+	txresp
+
+	rxreq
+	txresp
+} -start
+
+varnish v1 -vcl {
+	import backend_dyn from "${vmod_topbuild}/src/.libs/libvmod_backend_dyn.so";
+
+	backend dummy { .host = "${bad_ip}"; }
+
+	sub vcl_init {
+		if (!backend_dyn.create(name="be", host="${s1_addr}",
+					port="${s1_port}", max_connections=1)) {
+			return(fail);
+		}
+	}
+
+	sub vcl_recv {
+		set req.backend_hint = backend_dyn.by_name("be");
+	}
+} -start
+
+client c1 {
+	txreq -url "/foo"
+	rxresp
+	expect resp.status == 200
+
+	txreq -url "/bar"
+	rxresp
+	expect resp.status == 200
+} -run
+
+varnish v1 -expect backend_busy == 0
+varnish v1 -expect MAIN.n_backend == 2
+varnish v1 -expect VBE.vcl1.be.happy == 0