Build/Test Tools: Let `WP_UnitTestCase_Base` extend the namespaced version of...

Build/Test Tools: Let `WP_UnitTestCase_Base` extend the namespaced version of PHPUnit's test case class.

With the minimum PHP version requirement set to 5.6 we're now able to use namespaces by default. This replaces `PHPUnit_Framework_TestCase` with `PHPUnit\Framework\TestCase` for `WP_UnitTestCase_Base` to solve autocompletion issues with code editors when using more recent PHPUnit versions. `PHPUnit\Framework\TestCase` is available since PHPUnit 5.4 and now the minimum required PHPUnit version.

Fixes #50236.
Built from https://develop.svn.wordpress.org/trunk@47880


git-svn-id: https://core.svn.wordpress.org/trunk@47654 1a063a9b-81f0-0310-95a4-ce76da25c4cd