HG-279 Fix bug with displayed "Next page" button in ArticleComments component

There is a bug in Mercury and `Next page` button is visible in `ArticleComments` component even if there are no more pages with article comments.

Changes in this pull request fix it and introduce unit tests for the case.
as if you have expect(X) you have to have X number of tests
but if you don't call expect, it should as well,
unless the test here are async