Skip clipboard test on *nix systems if headless.

The clipboard is a matter for the X server.

IPython/lib/tests/test_clipboard.py

@@ -1,20 +1,22 b''
 import nose.tools as nt
 
 from IPython.core.error import TryNext
 from IPython.lib.clipboard import ClipboardEmpty
+from IPython.testing.decorators import skip_if_no_x11
 from IPython.utils.py3compat import unicode_type
 
+@skip_if_no_x11
 def test_clipboard_get():
     # Smoketest for clipboard access - we can't easily guarantee that the
     # clipboard is accessible and has something on it, but this tries to
     # exercise the relevant code anyway.
     try:
         a = get_ipython().hooks.clipboard_get()
     except ClipboardEmpty:
         # Nothing in clipboard to get
         pass
     except TryNext:
         # No clipboard access API available
         pass
     else:
         nt.assert_is_instance(a, unicode_type)