Add unit tests for CommandInsertLink fix

Add comprehensive unit tests to verify the fix prevents AttributeError
  when accessing non-existent nodes.

src/Mod/Assembly/TestAssemblyWorkbench.py

@@ -24,7 +24,9 @@
 import TestApp
 
 from AssemblyTests.TestCore import TestCore
+from AssemblyTests.TestCommandInsertLink import TestCommandInsertLink
 
 
 # Use the modules so that code checkers don't complain (flake8)
 True if TestCore else False
+True if TestCommandInsertLink else False