tests: Add a titlebar integration suite

It doesn't do all that much, but will prevent #2419 from regressing

tests/test-titlebar.lua

@@ -0,0 +1,94 @@
+local runner = require("_runner")
+local titlebar = require("awful.titlebar")
+local rules = require("awful.rules")
+local spawn = require("awful.spawn")
+
+local tiny_client = {"lua", "-e", [[
+local Gtk, class = require('lgi').require('Gtk'), 'client'
+Gtk.init()
+window = Gtk.Window {default_width=100, default_height=100, title='title'}
+window:set_wmclass(class, class)
+local app = Gtk.Application {}
+function app:on_activate()
+    window.application = self
+    window:show_all()
+end
+app:run {''}
+]]}
+
+-- Use the test client props
+rules.rules = {}
+
+-- Too bad there's no way to disconnect the rc.lua request::titlebars function
+
+local steps = {
+    function()
+        assert(#client.get() == 0)
+        spawn(tiny_client)
+        return true
+    end,
+    function()
+        if #client.get() ~= 1 then return end
+
+        local c = client.get()[1]
+
+        -- The rules don't set any borders nor enable the titlebar
+        assert(not c._request_titlebars_called)
+        assert(c.width == 100 and c.height == 100)
+
+        -- Should create the top titlebar
+        titlebar.toggle(c, "top")
+
+        assert(c._request_titlebars_called)
+
+        local h = c.height
+        assert(h > 100)
+
+        -- Should do nothing, there is no titlebar at the bottom by default
+        titlebar.toggle(c, "bottom")
+        assert(h == c.height)
+
+        -- Should hide the titlebar
+        titlebar.toggle(c, "top")
+        assert(c.height == 100)
+
+        c:kill()
+
+        return true
+    end,
+    function()
+        if #client.get() ~= 0 then return end
+
+        spawn(tiny_client, {titlebars_enabled=true})
+
+        return true
+    end,
+    function()
+        if #client.get() ~= 1 then return end
+
+        local c = client.get()[1]
+
+        local h = c.height
+        assert(c.width == 100 and h > 100)
+        assert(c._request_titlebars_called)
+
+        titlebar.hide(c, "top")
+
+        assert(c.width == 100 and c.height == 100)
+
+        titlebar.hide(c, "top")
+
+        assert(c.width == 100 and c.height == 100)
+        titlebar.show(c, "top")
+
+        assert(c.width == 100 and c.height == h)
+
+        c:kill()
+
+        return true
+    end,
+}
+
+runner.run_steps(steps)
+
+-- vim: filetype=lua:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:textwidth=80