Add no-op set_keyframe() to AbstractBaseNode

The test manager calls set_keyframe() on all nodes, but it was only
defined on AssemblyNode. This caused 'solid root test' to fail on
LeafNode subclasses like Solid2Node with AttributeError.

Author: lfagundes

solid_node/node/base.py

@@ -135,6 +135,11 @@ def get_source_file(self):
     def time(self):
         raise NotImplementedError
 
+    def set_keyframe(self, time):
+        """Set a fixed time for keyframes and tests.
+        No-op for non-animated nodes; overridden by AssemblyNode."""
+        pass
+
     def assemble(self, root=None):
         """Renders this node and returns an optimized version
         with all operations applied"""