Remove Plone unit-testing tutorial link.

The link redirects to a restrict and old knowledge base document[1]. 
  - requires plone.org site authentication
  - it is old, refers to the 2.1 version.  

[1] - https://plone.org/documentation/kb-old/richdocument/unit-testing
Merge pull request #148 from ramiroluz/master

Remove Plone unit-testing tutorial link.