naughty: Add a way to revert to the legacy mode.

When `rc.lua` will use the new notification API by default, we need
a way to revert back to the lagacy mode to be able to test it.

lib/naughty/core.lua

@@ -415,6 +415,10 @@ function naughty.get__has_preset_handler()
     return conns["request::preset"] and #conns["request::preset"] > 0 or false
 end
 
+function naughty._reset_display_handlers()
+    conns["request::display"] = nil
+end
+
 --- Set new notification timeout.
 --
 -- This function is deprecated, use `notification:reset_timeout(new_timeout)`.

tests/test-naughty-legacy.lua

@@ -11,6 +11,9 @@ local GLib      = require("lgi"        ).GLib
 local gpcall    = require("gears.protected_call")
 local dwidget   = require("naughty.widget._default")
 
+-- Bypass the new rc.lua and force the legacy mode again.
+naughty._reset_display_handlers()
+
 -- This module test deprecated APIs
 require("gears.debug").deprecate = function() end