make SkinTestCase more extendable

SkinTestCase which is part of trinidad-impl/test, works with TestSkin enum which does not offer flexibility. The change proposed is to add an overloaded method for visiting skins where the skin resource path can be passed as a String. This will enable test cases extending SkinTestCase to work with any skin of their choice.

The proposed change is to add this method in SkinTestCase:
/**

• Performs a visit of all style sheet nodes in the specified test
• skin. This overloaded method is useful in extending SkinTestCase to
• create more test cases since TestSkin does not provide all the skins
• that test needs.
  */
  protected final void visitStyleSheets(String skinResourcePath, StyleSheetVisitor visitor) { StyleSheetDocument document = SkinTestUtils.parseSkin(skinResourcePath); StyleSheetVisitUtils.visitStyleSheets(document.getStyleSheetsAsCollection(), visitor); }