In Behat How to Stage Link References in Table Nodes
This article from nuvole.org helped me resolve the same issue you describe.  From the article:


  A Link field in Drupal offers quite a wide range of options, such as
  an optional link title or internal/external URLs. We can use the
  following notation to work with links in our test scenarios:

When I am viewing a "post" content:
| title       | Post title                                              |
| field_link1 | http://nuvole.org                                       |
| field_link2 | Link 1 - http://nuvole.org                              |
| field_link3 | Link 1 - http://nuvole.org, Link 2 - http://example.com |

  
  As you can see we use always the same pattern: a
  dash - to separate parts of the same field value and a comma , to
  separate multiple field values.


The one note that I would add to the article is how to handle a link that has a URL, but not a title.  To handle that scenario, I used the following format:

When I am viewing a "post" content:
| link_with_text    | My link text - http://url.com |
| link_without_text | '' - https://google.com    |