The old version of the `ftintitle.contains_feat` function could lead to
false positives by matching words like "and" and "with" in the title,
even if there was no "feat. X" part.
With this commit, the `for_artist` keyword is explicitly passed to the
`plugins.feat_tokens` function to disable these matches when matching a
title (and not an artist).

The unit tests for the `ftintitle.contains_feat` function are now split
up for artist and title matching.

Add references to the ftintitle plugin for the bug fixes beetbox#5441 and beetbox#5436
that are related to this plugin.

Remove the optional `for_artist` keyword in the
`ftintitle.contains_feat` function and hardcode it to False, since it is
always used like this in the ftintitle plugin.

Since the `for_artist` keyword has been removed from
`ftintitle.contains_feat`, the unit tests need to be updated.
This includes the deletion of the test cases that test the
`for_artist=True` delimiters.

Remove redundant unit tests for the `ftintitle.cotains_feat` function