tests/test-xdg.t

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

The log files were being `cat'd` in `test-patchbomb-tls.t`, and causing
gratuitous failures.  Since `sys.stdout` is being written to with `str` instead
of `bytes`, use a `io.TextIOWrapper` to change the EOL, like 2924676d4728.

#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