patch 8.2.1344: Vim9: No test for trying to redefine global function

Problem:    Vim9: No test for trying to redefine global function.
Solution:   Add a test.

src/testdir/test_vim9_func.vim

@@ -159,6 +159,19 @@ def Test_nested_global_function()
       delfunc g:Inner
   END
   CheckScriptSuccess(lines)
+
+  lines =<< trim END
+      vim9script
+      def Outer()
+          def g:Inner(): string
+              return 'inner'
+          enddef
+      enddef
+      defcompile
+      Outer()
+      Outer()
+  END
+  CheckScriptFailure(lines, "E122:")
 enddef
 
 def Test_global_local_function()