SITE: Improvements Add white box unit test description.

  This commit is for the content changes only, not the output
  It adds the section "Validate contributions using the white box tests"
  to the "Contribute/How Can I Contribute?" link.

  It includes:
    A description of the test source layout.
    How to run he tests
    Why one may need to update the VCR fixtures?
    An example of updating the VCR fixtures.
TrackedAt: http://tracker.deltacloud.org/patch/f70075cdbbf849ef47c4b708e29e87cc95bca8fd