patch 8.1.0092: prompt buffer test fails

Problem:    Prompt buffer test fails.
Solution:   Set 'nomodified' before closing the window. (Ozaki Kiichi,
            closes #3051
diff --git a/src/testdir/test_prompt_buffer.vim b/src/testdir/test_prompt_buffer.vim
index a6269de..1b8a1ec 100644
--- a/src/testdir/test_prompt_buffer.vim
+++ b/src/testdir/test_prompt_buffer.vim
@@ -24,6 +24,8 @@
   call writefile([
 	\ 'func TextEntered(text)',
 	\ '  if a:text == "exit"',
+	\ '    " Reset &modified to allow the buffer to be closed.',
+	\ '    set nomodified',
 	\ '    stopinsert',
 	\ '    close',
 	\ '  else',