For me, because of the way workbench is setup, the test cases can only extend from PHPUnit_Framework_TestCase. So I run the test cases from the app folder instead of the package folder.
I choose the second option as testbench heavily depends on the particular Laravel version and seems a bit heavy to set up. What I did is configure the phpunit.xml under the main app to include the package tests folder as a test suite:
<testsuite name="MyPack">
<directory>./workbench/winponta/mypack/tests/</directory>
</testsuite>
And then run:
$ phpunit --testsuite=MyPack
Using this method, we can still extends from \Illuminate\Foundation\Testing\TestCase.
I found this solution at http://ngo-hung.com/blog/2014/04/16/laravel-unit-testing-in-workbench-note