dtoc: Add some tests for the fdt module

At present this module is tested via the dtoc tests. This is a bit painful
since the tests are at a higher level and so failures are more difficult
to diagnose.

Add some tests that exercise the fdt module directly.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/dtoc/fdt.py b/tools/dtoc/fdt.py
index d08b0b5..5cde8c1 100644
--- a/tools/dtoc/fdt.py
+++ b/tools/dtoc/fdt.py
@@ -138,6 +138,7 @@
         else:
             return TYPE_INT, val
 
+    @classmethod
     def GetEmpty(self, type):
         """Get an empty / zero value of the given type
 
@@ -335,12 +336,19 @@
         """
         return self._fdt
 
-    def CheckErr(errnum, msg):
+    def GetFdtObj(self):
+        """Get the contents of the FDT
+
+        Returns:
+            The FDT contents as a libfdt.Fdt object
+        """
+        return self._fdt_obj
+
+    def CheckErr(self, errnum, msg):
         if errnum:
             raise ValueError('Error %d: %s: %s' %
                 (errnum, libfdt.fdt_strerror(errnum), msg))
 
-
     def GetProps(self, node):
         """Get all properties from a node.