The aim is to be able to write phpunit assertions for phpunit 6 and then be able to run these for Magento 2.3 and earlier (more accurately their associated phpunit versions).

Install with

composer require fooman/phpunit-assert-bridge --dev

and then instead of \PHPUnit\Framework\Assert or \PHPUnit_Framework_Assert use Fooman\PhpunitAssertBridge\CompatAssert in both cases.

<?php

use Fooman\PhpunitAssertBridge\CompatAssert;

class MyTestCaseAssertion extends extends \Magento\Mtf\Constraint\AbstractConstraint
{

    public function processAssert(
    	...
	) {
    	CompatAssert::assertEquals('expected', $actual);			
	}

}