patch 8.2.0482: channel and sandbox code not sufficiently tested

Problem:    Channel and sandbox code not sufficiently tested.
Solution:   Add more tests. (Yegappan Lakshmanan, closes #5855)
diff --git a/src/testdir/test_prompt_buffer.vim b/src/testdir/test_prompt_buffer.vim
index fb5d134..39cce24 100644
--- a/src/testdir/test_prompt_buffer.vim
+++ b/src/testdir/test_prompt_buffer.vim
@@ -121,6 +121,11 @@
   call feedkeys("\<CR>\<C-C>", 'xt')
   call assert_true(v:true)
 
+  call assert_fails("call prompt_setcallback(bufnr(), [])", 'E921:')
+  call assert_equal(0, prompt_setcallback({}, ''))
+  call assert_fails("call prompt_setinterrupt(bufnr(), [])", 'E921:')
+  call assert_equal(0, prompt_setinterrupt({}, ''))
+
   delfunc MyPromptCallback
   bwipe!
 endfunc