D-Bus testing: use 'temp-test-dbus' as name of directory created for testing

'temp-test-dbus' is a bit more suitable than 'xdg-root' because
* it ties the directory to the script which creates it
* 'temp' implies that it holds no important data
* it is used for various files ('xdg_root' in the Python source
  is a bit misleading)

test/test-dbus.py

@@ -50,7 +50,8 @@ loop = gobject.MainLoop()
 debugger = "" # "gdb"
 server = ["syncevo-dbus-server"]
 monitor = ["dbus-monitor"]
-xdg_root = "xdg-root"
+# primarily for XDG files, but also other temporary files
+xdg_root = "temp-test-dbus"
 configName = "dbus_unittest"
 
 def property(key, value):