Add a debug.no_backend() function to simulate a director failing to

pick a backend.

lib/libvmod_debug/vmod.vcc

@@ -59,6 +59,10 @@ $Function STRING blob2hex(BLOB)
 
 Hexdump a blob
 
+$Function BACKEND no_backend()
+
+Fails at backend selection
+
 $Object obj(STRING) 
 
 Test object

lib/libvmod_debug/vmod_debug.c

@@ -129,3 +129,12 @@ vmod_blob2hex(const struct vrt_ctx *ctx, VCL_BLOB b)
 	VRT_priv_fini(b);
 	return (s);
 }
+
+VCL_BACKEND
+vmod_no_backend(const struct vrt_ctx *ctx)
+{
+	
+	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
+	return (NULL);
+}
+