Add missing status type 6 (warning)

[theseer]

Implementing phpunit/#1802 added a new status type which somehow never made it into the XML Coverage Report.
That lead to the following error condition:

     [exec] PHP Notice:  Undefined offset: 6 in phar:///usr/bin/phpunit/php-code-coverage/Report/Xml/Tests.php on line 43
     [exec] PHP Stack trace:
     [exec] PHP   1. {main}() /usr/bin/phpunit:0
     [exec] PHP   2. PHPUnit_TextUI_Command::main() /usr/bin/phpunit:569
     [exec] PHP   3. PHPUnit_TextUI_Command->run() phar:///usr/bin/phpunit/phpunit/TextUI/Command.php:113
     [exec] PHP   4. PHPUnit_TextUI_TestRunner->doRun() phar:///usr/bin/phpunit/phpunit/TextUI/Command.php:162
     [exec] PHP   5. SebastianBergmann\CodeCoverage\Report\Xml\Facade->process() phar:///usr/bin/phpunit/phpunit/TextUI/TestRunner.php:583
     [exec] PHP   6. SebastianBergmann\CodeCoverage\Report\Xml\Facade->processTests() phar:///usr/bin/phpunit/php-code-coverage/Report/Xml/Facade.php:52
     [exec] PHP   7. SebastianBergmann\CodeCoverage\Report\Xml\Tests->addTest() phar:///usr/bin/phpunit/php-code-coverage/Report/Xml/Facade.php:210

This PR should fix that by adding the missing status type