patch 8.2.0615: regexp benchmark stest is old style

Problem:    Regexp benchmark stest is old style.
Solution:   Make it a new style test.  Fix using a NULL list.  Add more tests.
            (Yegappan Lakshmanan, closes #5963)

src/testdir/test_prompt_buffer.vim

@@ -148,6 +148,7 @@ func Test_prompt_buffer_edit()
   call assert_beeps("normal! \<C-A>")
   call assert_beeps("normal! \<C-X>")
   close!
+  call assert_equal(0, prompt_setprompt([], ''))
 endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab