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: #17299

Signed-off-by: Phạm Bình An <phambinhanctb2004@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/tutor/tutor.tutor

@@ -17,15 +17,8 @@ Table of contents:
 
 ## SETTING UP *setting-up*
 
-First, you'll need to enable "debug" mode
-~~~ cmd
-    :let g:tutor_debug = 1
-~~~
-This will allow saving changes to the tutor files and will disable conceals, so
-you can more easily check your changes.
-
-After this, create a new .tutor file (we will be practicing on this very file, so you
-don't need to do this now):
+Create a new .tutor file (we will be practicing on this very file, so you don't
+need to do this now):
 ~~~ cmd
     :e new-tutorial.tutor
 ~~~