Adjust about page tests

No Issue
- Wait for list item element to appear before starting tests
  to make sure template partial has been rendered.
- Remove test for 20 list items as it's possible that there
  could be fewer than 20 contributors for during the window.
  Instead just test to make sure the list has at least one item.
Merge pull request #4718 from jaswilli/about-tests

Adjust about page tests