patch 8.2.1761: Vim9: Some tests are still using :let

Problem:    Vim9: Some tests are still using :let.
Solution:   Change more declarations to use :var.
diff --git a/src/testdir/test_vim9_assign.vim b/src/testdir/test_vim9_assign.vim
index eddc8c5..5d6dec1 100644
--- a/src/testdir/test_vim9_assign.vim
+++ b/src/testdir/test_vim9_assign.vim
@@ -3,6 +3,9 @@
 source check.vim
 source vim9.vim
 
+" TODO: remove later
+let v:disallow_let = 1
+
 let s:appendToMe = 'xxx'
 let s:addToMe = 111
 let g:existing = 'yes'