patch 8.2.2088: Vim9: script test sometimes fails

Problem:    Vim9: script test sometimes fails.
Solution:   Unlet variables.
diff --git a/src/testdir/test_vim9_script.vim b/src/testdir/test_vim9_script.vim
index dc62705..794a36e 100644
--- a/src/testdir/test_vim9_script.vim
+++ b/src/testdir/test_vim9_script.vim
@@ -1945,6 +1945,7 @@
         slist->add(t:tabvar)
       endfor
       assert_equal(['global', 'buf', 'win', 'tab', '1', '2', '3', '4'], slist)
+      unlet! g:globalvar b:bufvar w:winvar t:tabvar
   END
   CheckDefAndScriptSuccess(lines)