Add a test to confirm disabling ESI with standard VCL means.

resp.do_esi in VCL 4.1, req.esi in 4.0.

src/tests/disable_esi.vtc

+varnishtest "disable ESI with standard VCL (resp.do_esi or req.esi)"
+
+server s1 {
+	rxreq
+	txresp -body {<esi:include src="/body"/>}
+} -start
+
+server s2 {
+	rxreq
+	expect req.url == "/body"
+	txresp -body {foo}
+} -start
+
+varnish v1 -vcl+backend {
+	import ${vmod_pesi};
+
+	sub vcl_backend_fetch {
+		if (bereq.url == "/") {
+			set bereq.backend = s1;
+		}
+		else {
+			set bereq.backend = s2;
+		}
+	}
+
+	sub vcl_backend_response {
+		set beresp.do_esi = true;
+	}
+
+	sub vcl_deliver {
+		if (req.http.Disable) {
+			set resp.do_esi = false;
+		}
+		pesi.activate();
+	}
+} -start
+
+client c1 {
+	txreq
+	rxresp
+	expect resp.status == 200
+	expect resp.body == "foo"
+
+	txreq -hdr "Disable: 1"
+	rxresp
+	expect resp.status == 200
+	expect resp.body == {<esi:include src="/body"/>}
+} -run
+
+varnish v1 -syntax 4.0 -vcl {
+	import ${vmod_pesi};
+	backend b { .host = "${bad_ip}"; }
+
+	sub vcl_deliver {
+		if (req.http.Disable) {
+			set req.esi = false;
+		}
+		pesi.activate();
+	}
+}
+
+client c1 -run