patch 8.2.1351: Vim9: no proper error if using namespace for nested function

Problem:    Vim9: no proper error if using namespace for nested function.
Solution:   Specifically check for a namespace. (closes #6582)
diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index 85fd8ff..1ccaeb3 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -131,6 +131,8 @@
   CheckDefFailure(['def Nested(arg: string)', 'enddef', 'Nested()'], 'E119:')
 
   CheckDefFailure(['func Nested()', 'endfunc'], 'E1086:')
+  CheckDefFailure(['def s:Nested()', 'enddef'], 'E1075:')
+  CheckDefFailure(['def b:Nested()', 'enddef'], 'E1075:')
 enddef
 
 func Test_call_default_args_from_func()