Do not test FileIO('/dev/tty', 'w').seekable() in test_fileio

On OSX, _io.FileIO("/dev/tty", "w").isatty() is False
On Ubuntu, _io.FileIO("/dev/tty", "w").isatty() throws IOError: Illegal seek

Much like we see on other platforms, we cannot reliably
determine it is not seekable (or perhaps special in general).

Let's track in the related bug (http://bugs.jython.org/issue1945),
instead of test_fileio, which is more about the new IO support in
_io.FileIO.