Update tests for PHPUnit 7

Summary:
Use \PHPUnit\Framework\TestCase new class name.

Fix Uncaught SebastianBergmann\Timer\RuntimeException:
Cannot determine time at which the request started in
phar:///opt/phpunit7/php-timer/Timer.php:64 issue.

References:

• https://github.com/sebastianbergmann/phpunit/blob/6.0/ChangeLog-6.0.md#changed-1
• https://phpunit.de/manual/6.5/en/appendixes.annotations.html#appendixes.annotations.backupGlobals

Test Plan: arc unit

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1604