Disable CAS check when saving TestUser data.
During testing, we are not worried about data loss, so we can safely
bypass the CAS check when setting up a test fixture.
This change was added to address sporadic test failures like the following:
18:03:38 1) ApiEchoMarkReadTest::testMarkReadWithList
18:03:38 MWException: CAS update failed on user_touched for user ID '2' (read from slave); the version of the user to be saved is older than the current version.
18:03:38
18:03:38 /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/includes/user/User.php:3931
18:03:38 /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/tests/phpunit/includes/TestUser.php:83
18:03:38 /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/tests/phpunit/includes/api/ApiTestCase.php:30
18:03:38 /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/extensions/Echo/tests/phpunit/api/ApiEchoMarkReadTest.php:11
18:03:38 /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/tests/phpunit/MediaWikiTestCase.php:370
Bug: T131178
Change-Id: I99b43e0db85bc2c1cd335c82971df4e95520d34b