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);
}
}