wrap computation intensive functions in hook free env when makeing coverage test

so that Travis CI job won't fail because of timeout.

.luacov

@@ -2,6 +2,10 @@
 -- project folder as '.luacov' for project specific configuration
 -- @class module
 -- @name luacov.defaults
+
+-- global flag to indicate coverage test
+LUACOV = true
+
 return {
 
   -- default filename to load for config options if not provided

spec/unit/commonrequire.lua

@@ -21,3 +21,25 @@ Input.dummy = true
 -- turn on debug
 local DEBUG = require("dbg")
 --DEBUG:turnOn()
+
+-- remove debug hooks in wrapped function for better luacov performance
+if LUACOV then
+    local function hook_free_call(callback)
+        local hook, mask, count = debug.gethook()
+        debug.sethook()
+        local res = callback()
+        debug.sethook(hook, mask)
+        return res
+    end
+
+    local UIManager = require("ui/uimanager")
+    local uimanager_run = UIManager.run
+    function UIManager:run()
+        hook_free_call(function() return uimanager_run(UIManager) end)
+    end
+
+    local screen_shot = Screen.shot
+    function Screen:shot(filename)
+        hook_free_call(function() return screen_shot(Screen, filename) end)
+    end
+end