Add a slide on feature tests

[bernhardmgruber]

We have this table on the slides:

However, testing for a specific feature should be done via feature test macros. We should add a slide explaining how this is done.

[stale]

This issue or pull request has been automatically marked as stale because it has not had recent activity. Please manually close it, if it is no longer relevant, or ask for help or support to help getting it unstuck. Let me bring this to the attention of @klieret @wdconinc @michmx for now.