Some tests do not run standalone due to dbal dependency on global $cache

Example:

Fatal error: Call to a member function sql_exists() on a non-object in /home/phpdev/test-phpbb/phpBB/includes/db/dbal.php on line 259

Call Stack:
0.0002 322380 1.

() /home/pie/apps/git-phpunit-bundle/phpunit.php:0
0.0052 759028 2. PHPUnit_TextUI_Command::main() /home/pie/apps/git-phpunit-bundle/phpunit.php:44
0.0052 759512 3. PHPUnit_TextUI_Command->run() /home/pie/apps/git-phpunit-bundle/PHPUnit/TextUI/Command.php:125
0.0609 5073404 4. PHPUnit_TextUI_TestRunner->doRun() /home/pie/apps/git-phpunit-bundle/PHPUnit/TextUI/Command.php:187
0.0652 5419504 5. PHPUnit_Framework_TestSuite->run() /home/pie/apps/git-phpunit-bundle/PHPUnit/TextUI/TestRunner.php:325
0.0653 5419984 6. PHPUnit_Framework_TestSuite->run() /home/pie/apps/git-phpunit-bundle/PHPUnit/Framework/TestSuite.php:705
0.0653 5420376 7. PHPUnit_Framework_TestSuite->runTest() /home/pie/apps/git-phpunit-bundle/PHPUnit/Framework/TestSuite.php:745
0.0653 5420376 8. PHPUnit_Framework_TestCase->run() /home/pie/apps/git-phpunit-bundle/PHPUnit/Framework/TestSuite.php:772
0.0654 5420376 9. PHPUnit_Framework_TestResult->run() /home/pie/apps/git-phpunit-bundle/PHPUnit/Framework/TestCase.php:734
0.0666 5481536 10. PHPUnit_Framework_TestCase->runBare() /home/pie/apps/git-phpunit-bundle/PHPUnit/Framework/TestResult.php:649
2.1015 6137560 11. PHPUnit_Framework_TestCase->runTest() /home/pie/apps/git-phpunit-bundle/PHPUnit/Framework/TestCase.php:787
2.1015 6138812 12. ReflectionMethod->invokeArgs() /home/pie/apps/git-phpunit-bundle/PHPUnit/Framework/TestCase.php:925
2.1015 6138844 13. phpbb_dbal_write_sequence_test->test_write_sequence() /home/pie/apps/git-phpunit-bundle/PHPUnit/Framework/TestCase.php:925
2.1173 6730012 14. dbal->sql_fetchfield() /home/phpdev/test-phpbb/tests/dbal/write_sequence_test.php:53