Test that backend None works

src/tests/set_retrieve.vtc

@@ -123,11 +123,11 @@ logexpect l1 -v v1 -d 1 -g vxid -q "VCL_Error" {
 
 varnish v1 -vcl {
 	import ${vmod_re2};
-	backend b { .host = "${bad_ip}"; }
+	backend b0 None;
 
 	sub vcl_init {
 		new n = re2.set();
-		n.add("foo");
+		n.add("foo", backend = b0);
 		n.compile();
 	}
 
@@ -148,6 +148,7 @@ varnish v1 -vcl {
 		set resp.http.n-foo-n = n.nmatches();
 		set resp.http.n-foo-str = n.string();
 		set resp.http.n-foo-saved = n.saved(STR);
+		set resp.http.n-foo-backend = n.backend();
 	}
 
 }
@@ -183,6 +184,7 @@ client c1 {
 	expect resp.http.n-foo-n == 1
 	expect resp.http.n-foo-str == ""
 	expect resp.http.n-foo-saved == "false"
+	expect resp.http.n-foo-backend == ""
 } -run
 
 logexpect l1 -wait