Instead of extending PHPUnit_Framework_TestCase, extensions need to extend from PHPUnit\Framework\TestCase. The latter is available as a forwards-compat shim in PHPUnit 4.8.36 so it is safe to migrate over before PHPUnit 6 happens. Extensions using MediaWiki-CodeSniffer 16.0.0 will have had this automatically taken care of.
https://codesearch.wmflabs.org/search/?q=PHPUnit_Framework_TestCase&i=nope&files=&repos=