Fix tst_QStyleSheetStyle::toolTip().

Use the correct palette and enforce plain Windows style to prevent
the Vista style from clobbering the tooltip palette in polish().

Task-number: QTBUG-38183
Change-Id: Id19d548f818d801c4914a343e08207195c343888
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>
(cherry picked from qtbase/7a2547164d207119468abbfa4c0eb594948bb608)
Windows: Add missing colors to tooltip palette.

Task-number: QTBUG-38183
Change-Id: Ic3581cc87c0d61140624ddaf6fa85d682120ad4f
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>
(cherry picked from qtbase/9fc0c5466238eb8f43d2736d298ac36c65da529e)