Drupal registry table not populated with test class files
Do you by any chance have the contrib 'simpletest' module installed? (as opposed to the core simpletest module that comes bundled with drupal 7)

I had the same problem you have.

What fixed it for me was uninstalling simpletest. 
Than completely deleting the the contrib simpletest module.
Then reinstall simpletest (with the contrib module gone, this will install the core module)

And voila, my test classes started showing up in the registry and all is peachy :)