Only run GUI test when DISPLAY variable is set.

doc/todo.txt

@@ -1,2 +1 @@
-- Only run GUI tests when X server is available (ie. DISPLAY is set)
 - Port to Python 3.x (will not happen anytime soon though)

tests/__init__.py

@@ -183,6 +183,14 @@ def has_pyqt ():
 need_pyqt = _need_func(has_pyqt, "PyQT")
 
 
+@memoized
+def has_x11 ():
+    """Test if DISPLAY variable is set."""
+    return os.getenv('DISPLAY') is not None
+
+need_x11 = _need_func(has_x11, 'X11')
+
+
 @contextmanager
 def _limit_time (seconds):
     """Raises LinkCheckerInterrupt if given number of seconds have passed."""

tests/test_gui.py

@@ -16,13 +16,14 @@
 # 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 import unittest
 import sys
-from tests import need_pyqt
+from tests import need_pyqt, need_x11
 
 
 class TestGui (unittest.TestCase):
     """Test OMT GUI client."""
 
     @need_pyqt
+    @need_x11
     def test_gui (self):
         from PyQt4 import QtCore, QtGui, QtTest
         from linkcheck.gui import LinkCheckerMain