UI test framework Broken for extensions

Cache is not initiated correctly in php UI test framework. Is only needed when installing extensions, so problem was not noticed before.

 
Fatal error: Call to a member function getParameter() on a non-object in /home/travis/build/phpBB3/phpBB/phpbb/cache/driver/file.php on line 37
Call Stack:
0.0003 253008 1.

() /home/travis/build/phpBB3/phpBB/vendor/phpunit/phpunit/phpunit:0
0.0082 1265120 2. PHPUnit_TextUI_Command::main() /home/travis/build/phpBB3/phpBB/vendor/phpunit/phpunit/phpunit:52
0.0082 1271816 3. PHPUnit_TextUI_Command->run() /home/travis/build/phpBB3/phpBB/vendor/phpunit/phpunit/src/TextUI/Command.php:100
0.1819 13502336 4. PHPUnit_TextUI_TestRunner->doRun() /home/travis/build/phpBB3/phpBB/vendor/phpunit/phpunit/src/TextUI/Command.php:149
0.1937 13775512 5. PHPUnit_Framework_TestSuite->run() /home/travis/build/phpBB3/phpBB/vendor/phpunit/phpunit/src/TextUI/TestRunner.php:440
58.4095 100353280 6. PHPUnit_Framework_TestSuite->run() /home/travis/build/phpBB3/phpBB/vendor/phpunit/phpunit/src/Framework/TestSuite.php:722
58.4096 100356136 7. PHPUnit_Framework_TestSuite->run() /home/travis/build/phpBB3/phpBB/vendor/phpunit/phpunit/src/Framework/TestSuite.php:722
65.5626 103786968 8. PHPUnit_Framework_TestCase->run() /home/travis/build/phpBB3/phpBB/vendor/phpunit/phpunit/src/Framework/TestSuite.php:722
65.5627 103788176 9. PHPUnit_Framework_TestResult->run() /home/travis/build/phpBB3/phpBB/vendor/phpunit/phpunit/src/Framework/TestCase.php:724
65.5627 103789384 10. PHPUnit_Framework_TestCase->runBare() /home/travis/build/phpBB3/phpBB/vendor/phpunit/phpunit/src/Framework/TestResult.php:612
65.5948 105514696 11. phpbb_ui_test_case->setUp() /home/travis/build/phpBB3/phpBB/vendor/phpunit/phpunit/src/Framework/TestCase.php:764
65.5963 105788808 12. phpbb_ui_test_case->purge_cache() /home/travis/build/phpBB3/tests/test_framework/phpbb_ui_test_case.php:119
65.5963 105788888 13. phpbb_ui_test_case->get_cache_driver() /home/travis/build/phpBB3/tests/test_framework/phpbb_ui_test_case.php:349
65.5963 105789856 14. phpbb\cache\driver\file->__construct() /home/travis/build/phpBB3/tests/test_framework/phpbb_ui_test_case.php:341