Implement Unit Test for Task Page Provider

As part of Task Endpoint Pagination improvement a new page provider for the tasks endpoint was created (ELEMENTS-1126).
No tests were implemented since they cross with page provider tests (https://github.com/nuxeo/nuxeo-elements/blob/master/core/test/nuxeo-page-provider.test.js) and, probably, they will imply a small refactor to take advantage of both classes and shared code.