[tests] create DISPLAY environment variable to run tkinter tests

Change-Id: I912d9a5ad72231033c1fb6fddcfd773b359ec74f
---
M tests/tk_tests.py
1 file changed, 25 insertions(+), 5 deletions(-)

diff --git a/tests/tk_tests.py b/tests/tk_tests.py
index 58a283d..f94dffd 100755
--- a/tests/tk_tests.py
+++ b/tests/tk_tests.py
@@ -5,6 +5,7 @@
 #
 # Distributed under the terms of the MIT license.
 #
+import os
 import unittest
 from contextlib import suppress
 
@@ -12,12 +13,33 @@
 from tests.aspects import DefaultSiteTestCase, TestCase, require_modules
 
 
-class TestTkdialog(TestCase):
+class TkinterTestsBase(TestCase):
 
-    """Test Tkdialog."""
+    """TestCase base for Tkinter tests."""
 
     net = True
 
+    @classmethod
+    def setUpClass(cls):
+        """Set virtual display environment."""
+        super().setUpClass()
+        cls.env = os.environ.get('DISPLAY')
+        os.environ['DISPLAY'] = ':0.0'
+
+    @classmethod
+    def tearDownClass(cls):
+        """Restore the display environment value."""
+        if not cls.env:
+            del os.environ['DISPLAY']
+        else:
+            os.environ['DISPLAY'] = cls.env
+        super().tearDownClass()
+
+
+class TestTkdialog(TkinterTestsBase):
+
+    """Test Tkdialog."""
+
     def testTkdialog(self):
         """Test Tk dialog."""
         try:
@@ -27,12 +49,10 @@
             pywikibot.warning(e)
 
 
-class TestTkinter(DefaultSiteTestCase):
+class TestTkinter(TkinterTestsBase, DefaultSiteTestCase):
 
     """Test Tkinter."""
 
-    net = True
-
     def testTkinter(self):
         """Test Tkinter window."""
         root = tkinter.Tk()

