We wrote an extra NUL byte to the stream and, unsurprisingly, contents of its
buffer didn't match the original string resulting in the test failure.
Also get rid of a #if wxUSE_UNICODE.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63249 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775