vim-patch:9.2.0304: tests: test for 9.2.0285 doesn't always fail without the fix

Problem:  When the terminal is very large, test for 9.2.0285 doesn't
          trigger an ASAN error without the fix.
Solution: Use a window with fixed height (zeertzjq)

closes: vim/vim#19924

https://github.com/vim/vim/commit/b03970f41f17ce8f27b1c453784763527360d81b
(cherry picked from commit f8695fc529)

test/old/testdir/test_syntax.vim

@@ -994,9 +994,9 @@ func Test_syn_sync_grouphere_shorter_next_line()
       bar
     fi
   END
-  let lines = ['a']->repeat(50) + lines + ['a']->repeat(28 + winheight(0))
+  let lines = ['a']->repeat(50) + lines + ['a']->repeat(48)
 
-  new
+  20new
   call setline(1, lines)
   syn region shIf transparent
         \ start="\<if\_s" skip=+-fi\>+ end="\<;\_s*then\>" end="\<fi\>"