Fix message box tests on OS X.

From the QMessageBox::setWindowTitle docs:

     On Mac OS X, the window title is ignored (as required by the Mac OS X
     Guidelines).

tests/misc/test_msgbox.py

@@ -18,6 +18,8 @@
 
 """Tests for qutebrowser.misc.msgbox."""
 
+import sys
+
 import pytest
 
 from qutebrowser.misc import msgbox
@@ -38,6 +40,7 @@ def test_attributes(qtbot):
     box = msgbox.msgbox(parent=parent, title=title, text=text, icon=icon,
                         buttons=buttons)
     qtbot.add_widget(box)
+    if sys.platform != 'darwin':
         assert box.windowTitle() == title
     assert box.icon() == icon
     assert box.standardButtons() == buttons
@@ -80,6 +83,7 @@ def test_finished_signal(qtbot):
 def test_information(qtbot):
     box = msgbox.information(parent=None, title='foo', text='bar')
     qtbot.add_widget(box)
+    if sys.platform != 'darwin':
         assert box.windowTitle() == 'foo'
     assert box.text() == 'bar'
     assert box.icon() == QMessageBox.Information

tests/utils/test_error.py

@@ -18,6 +18,7 @@
 
 """Tests for qutebrowser.utils.error."""
 
+import sys
 import collections
 import logging
 
@@ -60,6 +61,7 @@ def test_err_windows(qtbot, qapp, pre_text, post_text, expected):
         w = qapp.activeModalWidget()
         try:
             qtbot.add_widget(w)
+            if sys.platform != 'darwin':
                 assert w.windowTitle() == 'title'
             assert w.icon() == QMessageBox.Critical
             assert w.standardButtons() == QMessageBox.Ok