Disable CAS check when saving TestUser data.

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