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)

src/testdir/term_util.vim

@@ -179,5 +179,9 @@ func Run_shell_in_terminal(options)
   return buf
 endfunc
 
+" Return concatenated lines in terminal.
+func Term_getlines(buf, lines)
+  return join(map(a:lines, 'term_getline(a:buf, v:val)'), '')
+endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab