Core Testing vs SimpleTest I have a query concerning the core Testing module and the SimpleTest module.

When I install a new site, the Testing module is available but disabled. When enabled, a list of tests are available for a whole range of resources such as nodes, users etc.

When I install SimpleTest these tests disappear along with the core Testing module. They are replaced by five example SimpleTests.

Could someone explain the differences between these two modules?

Also, is it possible to use these initial tests within SimpleTest?
The simpletest module has been integrated into Drupal 7 core. Since then the contrib simpletest module has not seen much development. See also this issue.

If you want to start writing tests for your own modules, enable the core simpletest module (called Testing in the UI) and do not install the contrib module. Also check out the examples module, it should give you an idea on how to add tests for custom code.