Fix test_non_gui_warning for Azure (and mplcairo).

anntzer · anntzer · commit f1bc93337cec · 2018-11-03T18:13:39.000+01:00
If on Linux and $DISPLAY is not set, then the non-gui warning is not
emitted.

We didn't notice that on Travis as $DISPLAY was indeed always set; but
it's not on Azure.
diff --git a/lib/matplotlib/tests/test_backend_bases.py b/lib/matplotlib/tests/test_backend_bases.py
@@ -58,11 +58,11 @@ def test_get_default_filename(tmpdir):
 
 
 @pytest.mark.backend('pdf')
-@pytest.mark.skipif('SYSTEM_TEAMFOUNDATIONCOLLECTIONURI' in os.environ,
-                    reason="this test fails an azure for unknown reasons")
-def test_non_gui_warning():
+def test_non_gui_warning(monkeypatch):
     plt.subplots()
 
+    monkeypatch.setitem(os.environ, "DISPLAY", ":999")
+
     with pytest.warns(UserWarning) as rec:
         plt.show()
         assert len(rec) == 1
