vtc: Coverage for withdrawn bgfetch

This adds coverage for a non-delivery transition from vcl_hit, where the
busy objcore would drop its sole reference in the event of a grace hit.
The lack of coverage was visible in the gcov dashboard:

    669   580   if (busy != NULL) {
    670   0     	(void)HSH_DerefObjCore(wrk, &busy, 0);
    671   0     	VRY_Clear(req);
    672   0     }

There should now be at least one pass inside this block.

Refs #4032

bin/varnishtest/tests/c00128.vtc

@@ -0,0 +1,29 @@
+varnishtest "Withdraw graced hit's busy objcore"
+
+server s1 {
+	rxreq
+	txresp
+} -start
+
+varnish v1 -vcl+backend {
+	sub vcl_backend_response {
+		set beresp.ttl = 1ms;
+	}
+	sub vcl_hit {
+		if (obj.ttl < 0s) {
+			return (fail);
+		}
+	}
+} -start
+
+client c1 {
+	txreq
+	rxresp
+	expect resp.status == 200
+
+	delay 0.01
+
+	txreq
+	rxresp
+	expect resp.status == 503
+} -run