The test generates a random UUID (here) then tests that converting lowercase letters to uppercase generates an exception. It is possible for the randomly-generated UUID to not contain any letters in one of the base-36 or base-16 representations tested, in which case the test will fail. While the chances of this happening are very low, it happened in https://integration.wikimedia.org/ci/job/mediawiki-extensions-hhvm-jessie/17492/console.
14:10:17 There was 1 failure: 14:10:17 14:10:17 1) Flow\Tests\Model\UUIDTest::testInvalidInputOnCreate with data set #2 ('0574757892939740563985') 14:10:17 Failed asserting that exception of type "\Flow\Exception\InvalidInputException" is thrown. 14:10:17 14:10:17 /home/jenkins/workspace/mediawiki-extensions-hhvm-jessie/src/tests/phpunit/MediaWikiTestCase.php:415 14:10:17 /home/jenkins/workspace/mediawiki-extensions-hhvm-jessie/src/maintenance/doMaintenance.php:111
The test should use static UUID inputs rather than generating one randomly.