patch 8.2.3461: cannot distinguish Normal and Terminal-Normal mode

Problem:    Cannot distinguish Normal and Terminal-Normal mode.
Solution:   Make mode() return "nt" for Terminal-Normal mode. (issue #8856)

src/testdir/test_functions.vim

@@ -874,6 +874,14 @@ func Test_mode()
   call assert_equal('c-ce', g:current_modes)
   " How to test Ex mode?
 
+  if has('terminal')
+    term
+    call feedkeys("\<C-W>N", 'xt')
+    call assert_equal('n', mode())
+    call assert_equal('nt', mode(1))
+    call feedkeys("aexit\<CR>", 'xt')
+  endif
+
   bwipe!
   iunmap <F2>
   xunmap <F2>