patch 8.2.2512: Vim9: compiling error test sometimes fails

Problem:    Vim9: compiling error test sometimes fails.
Solution:   use WaitForAssert() instead of sleeping for a bit. (Dominique
            Pellé, closes #7837)
diff --git a/src/testdir/test_vim9_script.vim b/src/testdir/test_vim9_script.vim
index 85eea13..996c357 100644
--- a/src/testdir/test_vim9_script.vim
+++ b/src/testdir/test_vim9_script.vim
@@ -755,7 +755,7 @@
 enddef
 
 def Test_error_in_nested_function()
-  # an error in a nested :function aborts executin in the calling :def function
+  # an error in a nested :function aborts executing in the calling :def function
   var lines =<< trim END
       vim9script
       def Func()