Test that the SheetReferences class can add references and give them
out
SheetReferences refs = new SheetReferences();
refs.addSheetReference("A", 0);
refs.addSheetReference("B", 1);
refs.addSheetReference("C", 3);
assertTrue("ref 0 == A", refs.getSheetName(0).equals("A"));
assertTrue("ref 1 == B", refs.getSheetName(1).equals("B"));
assertTrue("ref 3 == C", refs.getSheetName(3).equals("C"));