I used a fresh L4 install,
generated a workbench package
added the package service provider to config/app.php file under providers key
Then ran phpunit and test is not running.

Then I removed the service provider from the array and phpunit worked.

It seems something wrong with autoloading the extended class Illuminate\Foundation\Testing\TestCase
as if I extend the class with PHPUnit_Framework_TestCase then it does work.