Add failing test: c.startup_id should be nil when snid is disabled

When using `spawn` without startup notification support, the
`startup_id` property on the client should be nil.

This adds rxvt-unicode to the Travis build, because it supports startup
notifications, but xterm does not.  We should replace xterm with it in
the existing tests then later.

.travis.yml

@@ -31,7 +31,7 @@ install:
   - sudo apt-get install -y libcairo2-dev xmlto asciidoc libpango1.0-dev gperf luadoc libxcb-xtest0-dev libxcb-icccm4-dev libxcb-randr0-dev libxcb-keysyms1-dev libxcb-xinerama0-dev libxcb-image0-dev libev-dev libimlib2-dev libdbus-1-dev libxdg-basedir-dev libstartup-notification0-dev imagemagick libxcb1-dev libxcb-shape0-dev libxcb-util0-dev libxcursor-dev libx11-xcb-dev libxcb-cursor-dev libxcb-xkb-dev libxkbcommon-dev libxkbcommon-x11-dev
 
   # Deps for functional tests.
-  - sudo apt-get install -y dbus-x11 xterm xdotool xterm xvfb
+  - sudo apt-get install -y dbus-x11 xterm xdotool xterm xvfb rxvt-unicode
 
   # Install Lua (per env).
   - sudo apt-get install -y lib${LUANAME}-dev ${LUANAME} ${INSTALL_PKGS}

tests/test-spawn-snid.lua

@@ -0,0 +1,42 @@
+--- Tests for spawn's startup notifications.
+
+local spawn = require("awful.spawn")
+
+local manage_called, c_snid
+
+client.connect_signal("manage", function(c)
+  manage_called = true
+  c_snid = c.startup_id
+end)
+
+
+local ret, snid
+local steps = {
+  function(count)
+    if count == 1 then
+      ret, snid = spawn('urxvt', true)
+    elseif manage_called then
+      local c = client.get()[1]
+      assert(ret)
+      assert(snid)
+      assert(snid == c_snid)
+      return true
+    end
+  end,
+
+  -- Test that c.startup_id is nil for a client without startup notifications,
+  -- and especially not the one from the previous spawn.
+  function(count)
+    if count == 1 then
+      manage_called = false
+      ret, snid = spawn('urxvt', false)
+    elseif manage_called then
+      assert(ret)
+      assert(snid == nil)
+      assert(c_snid == nil, "c.startup_snid should be nil!")
+      return true
+    end
+  end
+}
+
+require("_runner").run_steps(steps)