Class '..\Unit\UnitTestCase' not found

I am upgrading laravel from 5.5.* to 5.6.0. 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:

Fatal error: Uncaught Error: Class 'PhiraterTest\Unit\UnitTestCase' not found in /home/vagrant/code/phirater-l51/tests/unit/Phirater/AdditionalCurrencies/CreateAdditionalCurrencyCommandHandlerTest.php:11 Stack trace: #0 /home/vagrant/code/phirater-l51/vendor/phpunit/phpunit/src/Util/Fileloader.php(64): include_once() #1 /home/vagrant/code/phirater-l51/vendor/phpunit/phpunit/src/Util/Fileloader.php(48): PHPUnit\Util\Fileloader::load('/home/vagrant/c...') #2 /home/vagrant/code/phirater-l51/vendor/phpunit/phpunit/src/Framework/TestSuite.php(325): PHPUnit\Util\Fileloader::checkAndLoad('/home/vagrant/c...') #3 /home/vagrant/code/phirater-l51/vendor/phpunit/phpunit/src/Framework/TestSuite.php(403): PHPUnit\Framework\TestSuite->addTestFile('/home/vagrant/c...') #4 /home/vagrant/code/phirater-l51/vendor/phpunit/phpunit/src/Runner/BaseTestRunner.php(65): PHPUnit\Framework\TestSuite->addTestFiles(Array) #5 /home/vagrant/code/phirater-l51/vendor/phpunit/phpunit/src/TextUI/Command.php(169): PHPUnit\Runner\BaseTestRunner->getTest('tes in /home/vagrant/code/phirater-l51/tests/unit/Phirater/AdditionalCurrencies/CreateAdditionalCurrencyCommandHandlerTest.php on line 11

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?

from Class '..\Unit\UnitTestCase' not found