In a Drupal 8 PHPunit migration test, iterate on rows. Warning, if migration is previously executed, no iteration will be made.

drupal8-phpubnit-migration.php

<?php
function testMyMigration() {
  $migration = $this->getMigration('MY_MIGRATION');

  $source = $migration->getSourcePlugin();
  $source->initializeIterator();
  for ($i=0 ; $i < $source->count(); $i++) {
    $source->rewind();
    $row = $source->current();
    $rows[] = $row;
  }
}