Add link to test project for csdb snippets with advanced tests

[delphidabbler]

If a snippet has advanced testing then it would be useful to be able to provide a link to a test project where the tests can be found.

[delphidabbler]

"Link" is to be TestURL that can links to the URL where either a project containing the test or demo is found or the URL of some custom test code for the snippet (e.g. in a Gist).