tests: Add an helper "before and after" method for the layouts

This will avoid a lot of copy paste as the
remove/set/swap/insert/add/remove_widgets/swap_widgets code
is identical beside the method name.

tests/examples/wibox/layout/template.lua

@@ -5,14 +5,15 @@ local wibox     = require( "wibox"         )
 local surface   = require( "gears.surface" )
 local color     = require( "gears.color"   )
 local beautiful = require( "beautiful"     )
+local unpack    = unpack or table.unpack -- luacheck: globals unpack (compatibility with Lua 5.1)
 
 -- Create a generic rectangle widget to show layout disposition
-local function generic_widget(text)
+local function generic_widget(text, col)
     return {
         {
             {
                 draw = function(_, _, cr, width, height)
-                    cr:set_source(color(beautiful.bg_normal))
+                    cr:set_source(color(col or beautiful.bg_normal))
                     cr:set_line_width(3)
                     cr:rectangle(0, 0, width, height)
                     cr:fill_preserve()
@@ -34,8 +35,63 @@ local function generic_widget(text)
     }
 end
 
+local names = {
+    "first"  ,
+    "second" ,
+    "third"  ,
+    "fourth" ,
+    "fifth"  ,
+    "sixth"  ,
+    "seventh",
+    "eighth" ,
+    "ninth"  ,
+}
+
+-- Generic template to create "before and after" for layout mutators
+local function generic_before_after(layout, layout_args, count, method, method_args)
+    local ls = {}
+
+    -- Create the layouts
+    for i=1, 2 do
+        local args = {layout = layout}
+
+        -- In case the layout change the array (it is technically not forbidden)
+        for k,v in pairs(layout_args) do
+            args[k] = v
+        end
+
+        local l = wibox.layout(args)
+        for j=1, count or 3 do
+            l:add(wibox.widget(generic_widget(names[j] or "N/A")))
+        end
+
+        ls[i] = l
+    end
+
+    -- Mutate
+    if type(method) == "function" then
+        method(ls[2], unpack(method_args or {}))
+    else
+        ls[2][method](ls[2], unpack(method_args or {}))
+    end
+
+    return wibox.layout {
+        {
+            markup = "<b>Before:</b>",
+            widget = wibox.widget.textbox,
+        },
+        ls[1],
+        {
+            markup = "<b>After:</b>",
+            widget = wibox.widget.textbox,
+        },
+        ls[2],
+        layout = wibox.layout.fixed.vertical
+    }
+end
+
 -- Let the test request a size and file format
-local widget, w, h = loadfile(file_path)(generic_widget)
+local widget, w, h = loadfile(file_path)(generic_widget, generic_before_after)
 
 -- Emulate the event loop for 10 iterations
 for _ = 1, 10 do