Add unit test for wrapping a margin container

This adds a test case where a `wibox.container.margin` with a
`wibox.widget.imagebox` as child is wrapped by a simple function call.

Check against regression in #3213.

Signed-off-by: Lucas Schwiderski <lucas@lschwiderski.de>

spec/wibox/container/margin_spec.lua

@@ -3,13 +3,60 @@
 -- @copyright 2017 Uli Schlachter
 ---------------------------------------------------------------------------
 
+local base = require('wibox.widget.base')
 local margin = require("wibox.container.margin")
+local imagebox = require("wibox.widget.imagebox")
 local utils = require("wibox.test_utils")
 
 describe("wibox.container.margin", function()
     it("common interfaces", function()
         utils.test_container(margin())
     end)
+
+    describe("composite widgets", function()
+        it("can be wrapped with child", function()
+            local widget_name = "test_widget"
+            local new = function()
+                local ret = base.make_widget_declarative {
+                    {
+                        id = "img",
+                        widget = imagebox,
+                    },
+                    widget = margin,
+                }
+
+                ret.widget_name = widget_name
+
+                return ret
+            end
+
+            local widget = base.make_widget_declarative {
+                widget = new,
+            }
+
+            assert.is.equal(
+                widget_name,
+                widget.widget_name,
+                "Widget name doesn't match"
+            )
+            local children = widget:get_children()
+            assert.is_not.Nil(children, "Widget doesn't have children")
+            assert.is.equal(
+                1,
+                #children,
+                "Widget should have exactly one child"
+            )
+            assert.is.True(
+                children[1].is_widget,
+                "Child widget should be a valid widget"
+            )
+            assert.is.equal(
+                widget.img,
+                children[1],
+                "Child widget should match the id accessor"
+            )
+        end)
+    end)
 end)
 
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