patch 9.0.0912: libvterm with modifyOtherKeys level 2 does not match xterm

Problem:    libvterm with modifyOtherKeys level 2 does not match xterm.
Solution:   Adjust key code escape sequences to be the same as what xterm
            sends in modifyOtherKeys level 2 mode.  Check the value of
            no_reduce_keys before using it.

src/testdir/keycode_check.vim

@@ -319,10 +319,10 @@ def DoTerm(name: string)
     ch_logfile('keylog-ignore', 'a')
     while 1
       sleep 100m
-      if !getchar(1)
+      if getchar(1) == 0
 	break
       endif
-      while getchar(1)
+      while getchar(1) != 0
 	getchar()
       endwhile
     endwhile