Add the vtc feature ignore_unknown_macro.

Closes: #2357

bin/varnishtest/tests/a00018.vtc

+varnishtest "feature ignore_unknown_macro"
+
+feature ignore_unknown_macro
+
+server s1 {
+	rxreq
+	expect req.http.Foo == "${foo}"
+	txresp -hdr "Bar: ${bar}"
+} -start
+
+varnish v1 -vcl {
+	backend default {
+		.host = "${s1_addr}";
+		.port = "${s1_port}";
+	}
+
+	sub vcl_deliver {
+		if (resp.http.Bar == "${bar}") {
+			set resp.http.Baz = "${baz}";
+		}
+	}
+} -start
+
+client c1 {
+	txreq -hdr "Foo: ${foo}"
+	rxresp
+	expect resp.status == 200
+	expect resp.http.Bar == "${bar}"
+	expect resp.http.Baz == "${baz}"
+} -run
+
+shell {
+	touch ${tmpdir}/tst
+	rm ${tmpdir}/tst
+}

bin/varnishtest/vtc.c

@@ -56,6 +56,7 @@ volatile sig_atomic_t	vtc_error;	/* Error encountered */
 int			vtc_stop;	/* Stops current test without error */
 pthread_t		vtc_thread;
 static struct vtclog	*vltop;
+static int		ign_unknown_macro = 0;
 
 /**********************************************************************
  * Macro facility
@@ -197,13 +198,18 @@ macro_expand(struct vtclog *vl, const char *text)
 		p += 2;
 		m = macro_get(p, q);
 		if (m == NULL) {
-			VSB_delete(vsb);
-			vtc_log(vl, 0, "Macro ${%.*s} not found", (int)(q - p),
-			    p);
-			return (NULL);
+			if (!ign_unknown_macro) {
+				VSB_delete(vsb);
+				vtc_log(vl, 0, "Macro ${%.*s} not found",
+				    (int)(q - p), p);
+				return (NULL);
+			}
+			VSB_printf(vsb, "${%.*s}", (int)(q - p), p);
+		}
+		else {
+			VSB_printf(vsb, "%s", m);
+			free(m);
 		}
-		VSB_printf(vsb, "%s", m);
-		free(m);
 		text = q + 1;
 	}
 	AZ(VSB_finish(vsb));
@@ -744,6 +750,9 @@ cmd_feature(CMD_ARGS)
 				good = 1;
 			else
 				vtc_stop = 1;
+		} else if (!strcmp(*av, "ignore_unknown_macro")) {
+			ign_unknown_macro = 1;
+			good = 1;
 		}
 		if (good)
 			continue;