patch 8.2.1766: Vim9: Some tests are still using :let

Problem:    Vim9: Some tests are still using :let.
Solution:   Change the last few declarations to use :var.

src/testdir/runtest.vim

@@ -355,6 +355,9 @@ func FinishTesting()
   qall!
 endfunc
 
+" TODO: remove later
+let v:disallow_let = 1
+
 " Source the test script.  First grab the file name, in case the script
 " navigates away.  g:testname can be used by the tests.
 let g:testname = expand('%')