Use relative paths when requiring xdo_test_helper.rb

The current directory is not in the $LOAD_PATH of more recent versions
of the Ruby interpreter (ruby1.9.1 and ruby2.0). So the location of
the file must be indicated.

Thanks to Cédric Boutillier <boutil@debian.org> for this fix.
pass file descriptors to child processes in test suite

From http://www.ruby-doc.org/core-2.1.2/IO.html#close_on_exec-3D-method:

> Ruby sets close-on-exec flags of all file descriptors by default
>  since Ruby 2.0.0.

This change ensures that the file descriptor used to retrieve the
window ID will be preserved across the exec, otherwise the test suite
hangs.

This change also rewrites the exec() call to avoid one layer of
possible command execution (the shell invocation by xterm itself still
remains).
Merge pull request #46 from dkg/master

clean up test suite to work with recent versions of ruby