Prevents the PHPUnit tests the TestCase and Illuminate\Foundation\Testing\TestCase classes

This would avoid the errors that are displayed when running unit tests in application based on phpunit.xml

Error displayed:
PHPUnit_Framework_AssertionFailedError : No tests found in class "Illuminate\Foundation\Testing\TestCase".
PHPUnit_Framework_AssertionFailedError : No tests found in class "TestCase".
Merge pull request #2583 from thallisphp/patch-1

Prevents the PHPUnit tests the Illuminate\Foundation\Testing\TestCase class