test: make chained clientmod for private tests

clientmods/mods.conf

@@ -46,3 +46,4 @@ load_mod_bookbot = true
 load_mod_invrefill = true
 load_mod_haxnotify = true
 load_mod_incrementaltp = true
+load_mod_test = true

clientmods/test/init.lua

@@ -0,0 +1,4 @@
+if minetest.settings:get_bool("test_chain") then
+    local prefix = minetest.get_modpath(minetest.get_current_modname())
+    dofile(prefix .. "/chain.lua")
+end

clientmods/test/settingtypes.txt

@@ -0,0 +1 @@
+test_chain (Load the chain file) bool false