3.0 testsuite cleanup

Cleanup and remove the custom testsuite infrastructure from CakePHP. Now that cakephp/app does not have the webrunner, the only use of this is for the TestShell. I don't think there is much point in keeping TestShell around as it doesn't provide much value over a well built `phpunit.xml` file.

I've done a few changes to Plugin baking to do the following:

* Generate a phpunit.xml file.
* Generate a Test/bootstrap.php file to help ensure the tests have a chance of running.

A few un-resolved problems with plugin baking that I want to fix in the future:

* Relative paths should be used in both the phpunit.xml and Test/bootstrap.php file.
* It would be really nice to append into the app's phpunit.xml and add the plugin suites.