patch 8.1.1848: 'langmap' is not used for CTRL-W command in terminal

Problem:    'langmap' is not used for CTRL-W command in terminal.
Solution:   Push the command in the typeahead buffer instead of the stuff
            buffer. (closes #4814)
diff --git a/src/testdir/test_terminal.vim b/src/testdir/test_terminal.vim
index 5496b63..e173b42 100644
--- a/src/testdir/test_terminal.vim
+++ b/src/testdir/test_terminal.vim
@@ -1686,6 +1686,13 @@
   call feedkeys("\<C-W>w", 'tx')
   call assert_equal(termwin, win_getid())
 
+  set langmap=xjyk
+  call feedkeys("\<C-L>x", 'tx')
+  call assert_equal(thiswin, win_getid())
+  call feedkeys("\<C-W>y", 'tx')
+  call assert_equal(termwin, win_getid())
+  set langmap=
+
   call feedkeys("\<C-L>gt", "xt")
   call assert_equal(3, tabpagenr())
   tabprev