tests/test-xdg.t

util: move common proxyobserver attributes to the base class

Fixes the following pytype warnings:

    line 791, in _writedata: No attribute 'logdata' on baseproxyobserver [attribute-error]
    line 792, in _writedata: No attribute 'logdataapis' on baseproxyobserver [attribute-error]
    line 793, in _writedata: No attribute 'fh' on baseproxyobserver [attribute-error]
    line 794, in _writedata: No attribute 'fh' on baseproxyobserver [attribute-error]
    line 799, in _writedata: No attribute 'logdataapis' on baseproxyobserver [attribute-error]
    line 800, in _writedata: No attribute 'fh' on baseproxyobserver [attribute-error]
    line 802, in _writedata: No attribute 'fh' on baseproxyobserver [attribute-error]
    line 803, in _writedata: No attribute 'name' on baseproxyobserver [attribute-error]
    line 805, in _writedata: No attribute 'fh' on baseproxyobserver [attribute-error]
    line 809, in _writedata: No attribute 'logdataapis' on baseproxyobserver [attribute-error]
    line 810, in _writedata: No attribute 'fh' on baseproxyobserver [attribute-error]
    line 814, in _writedata: No attribute 'fh' on baseproxyobserver [attribute-error]
    line 815, in _writedata: No attribute 'name' on baseproxyobserver [attribute-error]
    line 817, in _writedata: No attribute 'fh' on baseproxyobserver [attribute-error]

Differential Revision: https://phab.mercurial-scm.org/D7675

#if no-windows no-osx

  $ mkdir -p xdgconf/hg
  $ echo '[ui]' > xdgconf/hg/hgrc
  $ echo 'username = foobar' >> xdgconf/hg/hgrc
  $ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
  $ unset HGRCPATH
  $ hg config ui.username 2>/dev/null
  foobar

#endif