This PR was squashed before being merged into the 2.1.x-dev branch (closes #1514).

Discussion
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Fixed WebTestCase compatibility with PHPUnit 6+

Makes `WebTestCase` inherit from `PHPUnit\Framework\TestCase` instead of `PHPUnit_Framework_TestCase`.

Strictly speaking it's a BC break, but as the namespaced class is now available in PHPUnit 4.8.35, 5.4+ and 6.x it's probable safe.

Commits
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619018a7 Fixed WebTestCase compatibility with PHPUnit 6+