test the use of non-capturing parentheses with the sub() method and function

src/tests/non_capture_paren.vtc

@@ -29,6 +29,9 @@ varnish v1 -vcl+backend {
 		} else {
 			set beresp.status = 999;
 		}
+		set beresp.http.sub = barbaz.sub(beresp.http.foo, "\0\1\2");
+		set beresp.http.subf = re2.sub("(?:bar)(baz)", beresp.http.foo,
+		                               "\0\1\2");
 	}
 
 } -start
@@ -42,6 +45,8 @@ client c1 {
 	expect resp.http.foo0f == "barbaz"
 	expect resp.http.foo1f == "baz"
 	expect resp.http.foo2f == "fallback"
+	expect resp.http.sub == "barbazbaz"
+	expect resp.http.subf == "barbazbaz"
 } -run
 
 logexpect l1 -v v1 -d 1 -g vxid -q "VCL_Error" {