vim-patch:9.1.1384: still some problem with the new tutors filetype plugin

Problem:  still some problem with the new tutors filetype plugin
Solution: refactor code to enable/disable tutor mode into
          tutor#EnableInteractive() function, include a test
          (Phạm Bình An)

I find it annoying that Tutor's interactive mode is always on (or debug
mode is off) even when I open a tutor file with :edit command.
I think it makes more sense to make this "interactive mode":

- Always on when it is opened with :Tutor command
- Off otherwise

For more references, see `:help` feature, it is a much better than
:Tutor, since I don't have to run `:let g:help_debug = 1` just to be able
to edit and save a help file

Therefore, I remove `g:tutor_debug`

closes: vim/vim#17299

13bea589a2

Co-authored-by: Phạm Bình An <phambinhanctb2004@gmail.com>
(cherry picked from commit e01f196e44)

test/old/testdir/test_plugin_tutor.vim

@@ -0,0 +1,16 @@
+" Test for the new-tutor plugin
+
+func SetUp()
+  set nocompatible
+  runtime plugin/tutor.vim
+endfunc
+
+func Test_auto_enable_interactive()
+  Tutor
+  call assert_equal('nofile', &buftype)
+  call assert_match('tutor#EnableInteractive', b:undo_ftplugin)
+
+  edit Xtutor/Xtest.tutor
+  call assert_true(empty(&buftype))
+  call assert_match('tutor#EnableInteractive', b:undo_ftplugin)
+endfunc