[Test] Functional test for lua_extras two-phase loader

[Test] Functional test for lua_extras two-phase loader

Adds Functional.Cases.001_Merged.271_Lua_Extras with two cases:

* the deferred-selector regexp fires when the From-domain is present in
  the map captured by the selector factory;
* the same regexp stays silent when the From-domain is absent.

The companion lua_extras_test.lua stages a tree under TMPDIR with maps,
selectors and regexps subdirectories, then calls lua_extras.load_extras
on it. The selector entry is wrapped in lua_extras.deferred so the
factory captures rspamd_maps[name] at registration time, exercising the
maps -> selectors -> regexps phase 2 ordering and the re_selector
auto-binding into the regexp DSL.

Also wires the new lua file into merged.conf alongside selector_test.lua.