* Create an abstract `WP_UnitTestCase_Base` class to share between PHPUnit 7.x and older versions.
* Add a speed-trap loader to determine which `SpeedTrapListener` class needs to be loaded for the current PHPUnit version.
* Remove unnecessary `PHPUnit\Util\Test` and `PHPUnit_Util_Getopt` inheritances.
* Update Travis CI config to use PHPUnit 7.x for PHP 7.1, 7.2, and nightly PHP versions.
Props jipmoors, netweb, desrosj, ayeshrajans, soulseekah, SergeyBiryukov.
See #43218.
git-svn-id: https://develop.svn.wordpress.org/trunk@44701 602fd350-edb4-49c9-b593-d223f7449a82
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