tst_qdatetimeedit: call setCalendarPopup(false) when cleaning up

Tests could change this property, and since the majority of the tests
share testWidget instead of creating local instances, we should be
thorough when cleaning up.

Change-Id: I6039fb6857f4f788f809b1d1fd491b4ef34923ca
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

tests/auto/widgets/widgets/qdatetimeedit/tst_qdatetimeedit.cpp

@@ -457,6 +457,8 @@ void tst_QDateTimeEdit::cleanup()
     testWidget->setTimeSpec(Qt::LocalTime);
     testWidget->setSpecialValueText(QString());
     testWidget->setWrapping(false);
+    // Restore the default.
+    testWidget->setCalendarPopup(false);
 }
 
 void tst_QDateTimeEdit::constructor_qwidget()