Add a no_stevedore function for testing

lib/libvmod_debug/vmod.vcc

@@ -71,6 +71,10 @@ $Function BACKEND no_backend()
 
 Fails at backend selection
 
+$Function STEVEDORE no_stevedore()
+
+Fails at storage selection.
+
 $Object obj(STRING string="default", ENUM { one, two, three } number="one")
 
 Test object

lib/libvmod_debug/vmod_debug.c

@@ -184,6 +184,14 @@ vmod_no_backend(VRT_CTX)
 	return (NULL);
 }
 
+VCL_STEVEDORE __match_proto__(td_debug_no_stevedore)
+vmod_no_stevedore(VRT_CTX)
+{
+
+	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
+	return (NULL);
+}
+
 VCL_VOID __match_proto__(td_debug_rot52)
 vmod_rot52(VRT_CTX, VCL_HTTP hp)
 {