Use PHPUnit\Framework\TestCase instead of PHPUnit_Framework_TestCase

[carusogabriel]

I use the PHPUnit\Framework\TestCase notation instead of PHPUnit_Framework_TestCase while extending our TestCases. This will help us migrate to PHPUnit 6, that no longer support snake case class names.

I just need to bump PHPUnit version to >=4.8.35, that support this namespace.

[bfanger]

Thanks, but the 3.x branch already uses phpunit 6+

[bfanger]

Merged as 8bb17cf
Makes it possible to switch between master and the 3.x branch without having to install another phpunit version.

Thanks!