tests: The the mouse miss handler

tests/test-miss-handlers.lua

@@ -1,5 +1,6 @@
 -- Test set_{,new}index_miss_handler
 
+local mouse = mouse
 local class = tag
 local obj = class({})
 local handler = require("gears.object.properties")
@@ -30,4 +31,8 @@ assert(not obj.key)
 obj.key = 1337
 assert(obj.key == 1337)
 
+-- The the custom mouse handler
+mouse.foo = "bar"
+assert(mouse.foo == "bar")
+
 require("_runner").run_steps({ function() return true end })