patch 8.1.0030: stoping Vim running in a terminal may not work

Problem:    Stoping Vim running in a terminal may not work.
Solution:   Instead of sending <Esc> send CTRL-O.

src/testdir/test_prompt_buffer.vim

@@ -34,9 +34,12 @@ func Test_prompt_basic()
 	\ 'func TimerFunc(text)',
 	\ '  " Add the output above the current prompt.',
 	\ '  call append(line("$") - 1, "Result: \"" . a:text . "\"")',
+	\ '  " Reset &modified to allow the buffer to be closed.',
+	\ '  set nomodified',
 	\ 'endfunc',
 	\ '',
 	\ 'call setline(1, "other buffer")',
+	\ 'set nomodified',
 	\ 'new',
 	\ 'set buftype=prompt',
 	\ 'call prompt_setcallback(bufnr(""), function("TextEntered"))',