tests/test-xdg.t

tests: force `dumbhttp.py` to write its log file with '\n' on Windows

This wasn't causing obvious test failures, but it's the same fix as dbd2d56224d1
for `dummysmtpd.py`, and there's no sense in leaving this problem lying around.
(And upon further review, it might have been causing some non-obviously related
failures- see the next commit.)

#if no-windows

  $ 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