This renames the private `_next_token` method to `next_visitable_token`. It also removes irrelevant assertions from the unit test.
Follow-up to [59285].
Props dmsnell, jonsurrell, westonruter.
See #62269.
git-svn-id: https://develop.svn.wordpress.org/trunk@59364 602fd350-edb4-49c9-b593-d223f7449a82