patch 8.2.1772: cannot use CTRL-W <Down> to move out of a terminal window

Problem:    Cannot use CTRL-W <Down> to move out of a terminal window.
Solution:   Use special_to_buf() instead of mb_char2bytes(). (closes #7045)
diff --git a/src/testdir/test_terminal.vim b/src/testdir/test_terminal.vim
index 207ef9a..676d864 100644
--- a/src/testdir/test_terminal.vim
+++ b/src/testdir/test_terminal.vim
@@ -1379,6 +1379,23 @@
   set statusline=
 endfunc
 
+func Test_terminal_window_focus()
+  let winid1 = win_getid()
+  terminal
+  let winid2 = win_getid()
+  call feedkeys("\<C-W>j", 'xt')
+  call assert_equal(winid1, win_getid())
+  call feedkeys("\<C-W>k", 'xt')
+  call assert_equal(winid2, win_getid())
+  " can use a cursor key here
+  call feedkeys("\<C-W>\<Down>", 'xt')
+  call assert_equal(winid1, win_getid())
+  call feedkeys("\<C-W>\<Up>", 'xt')
+  call assert_equal(winid2, win_getid())
+
+  bwipe!
+endfunc
+
 func Api_drop_common(options)
   call assert_equal(1, winnr('$'))