patch 8.2.3841: Vim9: outdated TODO items, disabled tests that work

Problem:    Vim9: outdated TODO items, disabled tests that work.
Solution:   Remove TODO items, run tests that work now.  Check that a dict
            item isn't locked.

src/testdir/test_vim9_assign.vim

@@ -2017,6 +2017,22 @@ def Test_unlet()
    'defcompile',
    ], 'E1081:')
 
+  CheckScriptFailure([
+   'vim9script',
+   'def Delcount(dict: dict<any>)',
+   '  unlet dict.count',
+   'enddef',
+   'Delcount(v:)',
+   ], 'E742:')
+
+  CheckScriptFailure([
+   'vim9script',
+   'def DelChangedtick(dict: dict<any>)',
+   '  unlet dict.changedtick',
+   'enddef',
+   'DelChangedtick(b:)',
+   ], 'E795:')
+
   writefile(['vim9script', 'export var svar = 1234'], 'XunletExport.vim')
   var lines =<< trim END
     vim9script