Test adding (dynamic) backends with probe to already warm vcl

this tests d912ffe4

Tweaked:
	bin/varnishtest/tests/d00007.vtc

In 6.0 this bug doesn't exist and directors behave slightly differently
so the test case needed some adjustments to both show that a backend can
be added to an already warm VCL with a probe, and that requests succeed.

bin/varnishtest/tests/d00007.vtc

@@ -3,6 +3,10 @@ varnishtest "Test dynamic backends"
 server s1 {
 	rxreq
 	txresp
+	close
+	accept
+	rxreq
+	txresp
 } -start
 
 varnish v1 -vcl {
@@ -10,11 +14,18 @@ varnish v1 -vcl {
 
 	backend dummy { .host = "${bad_backend}"; }
 
+	probe pr {
+		.window = 1;
+		.threshold = 1;
+		.initial = 1;
+	}
+
 	sub vcl_init {
-		new s1 = debug.dyn("${s1_addr}", "${s1_port}");
+		new s1 = debug.dyn("${bad_backend}", "0");
 	}
 
 	sub vcl_recv {
+		s1.refresh("${s1_addr}", "${s1_port}", pr);
 		set req.backend_hint = s1.backend();
 	}
 } -start
@@ -26,3 +37,5 @@ client c1 {
 	rxresp
 	expect resp.status == 200
 } -run
+
+server s1 -wait

lib/libvmod_debug/vmod.vcc

@@ -129,7 +129,7 @@ $Function VOID fail()
 
 Function to fail vcl code.  (See also: RFC748)
 
-$Object dyn(STRING addr, STRING port)
+$Object dyn(STRING addr, STRING port, PROBE probe=0)
 
 Dynamically create a single-backend director, addr and port must not be empty.
 
@@ -137,7 +137,7 @@ $Method BACKEND .backend()
 
 Return the dynamic backend.
 
-$Method VOID .refresh(STRING addr, STRING port)
+$Method VOID .refresh(STRING addr, STRING port, PROBE probe=0)
 
 Dynamically refresh & (always!) replace the backend by a new one.
 

lib/libvmod_debug/vmod_debug_dyn.c

@@ -60,7 +60,7 @@ struct xyzzy_debug_dyn_uds {
 
 static void
 dyn_dir_init(VRT_CTX, struct xyzzy_debug_dyn *dyn,
-    VCL_STRING addr, VCL_STRING port)
+     VCL_STRING addr, VCL_STRING port, VCL_PROBE probe)
 {
 	struct addrinfo hints, *res = NULL;
 	struct suckaddr *sa;
@@ -75,6 +75,7 @@ dyn_dir_init(VRT_CTX, struct xyzzy_debug_dyn *dyn,
 	vrt.port = port;
 	vrt.vcl_name = dyn->vcl_name;
 	vrt.hosthdr = addr;
+	vrt.probe = probe;
 
 	memset(&hints, 0, sizeof(hints));
 	hints.ai_family = AF_UNSPEC;
@@ -116,7 +117,7 @@ dyn_dir_init(VRT_CTX, struct xyzzy_debug_dyn *dyn,
 
 VCL_VOID
 xyzzy_dyn__init(VRT_CTX, struct xyzzy_debug_dyn **dynp,
-    const char *vcl_name, VCL_STRING addr, VCL_STRING port)
+    const char *vcl_name, VCL_STRING addr, VCL_STRING port, VCL_PROBE probe)
 {
 	struct xyzzy_debug_dyn *dyn;
 
@@ -139,7 +140,7 @@ xyzzy_dyn__init(VRT_CTX, struct xyzzy_debug_dyn **dynp,
 
 	AZ(pthread_mutex_init(&dyn->mtx, NULL));
 
-	dyn_dir_init(ctx, dyn, addr, port);
+	dyn_dir_init(ctx, dyn, addr, port, probe);
 	XXXAN(dyn->dir);
 	*dynp = dyn;
 }
@@ -172,11 +173,11 @@ xyzzy_dyn_backend(VRT_CTX, struct xyzzy_debug_dyn *dyn)
 
 VCL_VOID
 xyzzy_dyn_refresh(VRT_CTX, struct xyzzy_debug_dyn *dyn,
-    VCL_STRING addr, VCL_STRING port)
+    VCL_STRING addr, VCL_STRING port, VCL_PROBE probe)
 {
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
 	CHECK_OBJ_NOTNULL(dyn, VMOD_DEBUG_DYN_MAGIC);
-	dyn_dir_init(ctx, dyn, addr, port);
+	dyn_dir_init(ctx, dyn, addr, port, probe);
 }
 
 static int