tests: hide warning from test-xdg.t

The test-xdg.t test uses the system hgrc because it unsets HGRCPATH. If the
system has an extension enabled that doesn't work with the development version
of Mercurial it prints an 'extension failed to load' warning. Let's just hide
that by piping stderr to /dev/null.

tests/test-xdg.t

@@ -1,11 +1,11 b''
 #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
+  $ hg config ui.username 2>/dev/null
   foobar
 
 #endif