get_discussion_posts does not include the 'messagetrust' field

In MDL-65252, as part of deprecation of "get_forum_discussion_posts" we had to change the test methods "test_trusted_text_enabled" and "test_trusted_text_disabled" after changing their usages of "get_forum_discussion_posts" to the recommended replacement method "get_discussion_posts". This was because the data we needed to assert was present, was not.  Namely, lines like this:

$this->assertEquals(1, $posts['posts'][0]['messagetrust']); 

That field just isn't present in the returns of the replacement method.

This issue is about working out whether or not that's expected. I'm not sure why it was there in the original method, but we certainly had tests covering it and the replacement should probably also return it (right?).