patch 8.2.0363: some Normal mode commands not tested

Problem:    Some Normal mode commands not tested.
Solution:   Add more tests. (Yegappan Lakshmanan, closes #5746)
diff --git a/src/testdir/test_prompt_buffer.vim b/src/testdir/test_prompt_buffer.vim
index 199c1c2..e13f9a6 100644
--- a/src/testdir/test_prompt_buffer.vim
+++ b/src/testdir/test_prompt_buffer.vim
@@ -124,3 +124,15 @@
   delfunc MyPromptCallback
   bwipe!
 endfunc
+
+" Test for editing the prompt buffer
+func Test_prompt_buffer_edit()
+  new
+  set buftype=prompt
+  normal! i
+  call assert_beeps('normal! dd')
+  call assert_beeps('normal! ~')
+  close!
+endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab