Class '..\Unit\UnitTestCase' not found

I am upgrading laravel from 5.4.* to 5.5.0. Composer install ran fine. I am trying to run unit tests upon which this error is popping up.

Class '..\Unit\UnitTestCase' not found

This is the stack trace of the error:

PHP Stack trace:
PHP   1. {main}() /home/vagrant/code/phirater-l51/vendor/phpunit/phpunit/phpunit:0
PHP   2. PHPUnit_TextUI_Command::main() /home/vagrant/code/phirater-l51/vendor/phpunit/phpunit/phpunit:52
PHP   3. PHPUnit_TextUI_Command->run() /home/vagrant/code/phirater-l51/vendor/phpunit/phpunit/src/TextUI/Command.php:116
PHP   4. PHPUnit_TextUI_TestRunner->getTest() /home/vagrant/code/phirater-l51/vendor/phpunit/phpunit/src/TextUI/Command.php:138
PHP   5. PHPUnit_Framework_TestSuite->addTestFiles() /home/vagrant/code/phirater-l51/vendor/phpunit/phpunit/src/Runner/BaseTestRunner.php:57
PHP   6. PHPUnit_Framework_TestSuite->addTestFile() /home/vagrant/code/phirater-l51/vendor/phpunit/phpunit/src/Framework/TestSuite.php:393
PHP   7. PHPUnit_Util_Fileloader::checkAndLoad() /home/vagrant/code/phirater-l51/vendor/phpunit/phpunit/src/Framework/TestSuite.php:316
PHP   8. PHPUnit_Util_Fileloader::load() /home/vagrant/code/phirater-l51/vendor/phpunit/phpunit/src/Util/Fileloader.php:36
PHP   9. include_once() /home/vagrant/code/phirater-l51/vendor/phpunit/phpunit/src/Util/Fileloader.php:52

My unit tests are in tests/ directory. My UnitTestCase class in extended by \TestCase class and TestCase class is extended by BrowserKitTestCase. What am i doing wrong here? What could be the solution?

via Chebli Mohamed