patch 8.2.1022: various parts of code not covered by tests

Problem:    Various parts of code not covered by tests.
Solution:   Add more tests. (Yegappan Lakshmanan, closes #6300)

src/testdir/test_prompt_buffer.vim

@@ -147,6 +147,11 @@ func Test_prompt_buffer_edit()
   call assert_beeps('normal! S')
   call assert_beeps("normal! \<C-A>")
   call assert_beeps("normal! \<C-X>")
+  " pressing CTRL-W in the prompt buffer should trigger the window commands
+  call assert_equal(1, winnr())
+  exe "normal A\<C-W>\<C-W>"
+  call assert_equal(2, winnr())
+  wincmd w
   close!
   call assert_equal(0, prompt_setprompt([], ''))
 endfunc