How can I load a migration in a KernelTest?

I'm trying to test a migration with a KernelTest, but I'm running into an issue when I try to load the migration plugin.

My migration is defined in my_module/migrations/my_migration.yml. In my KernelTest, I've listed my_module and migrate in the $modules static property.

In a custom form, I use the following to load a migration plugin:

/** @var \Drupal\migrate\Plugin\MigrationPluginManagerInterface $plugin_manager */
$plugin_manager = \Drupal::service('plugin.manager.migration');
$migration = $plugin_manager
  ->createInstance('MY_MIGRATION');
// Next, create MigrateExecutable, etc...

However, when I use this same code in my KernelTest, $migration is always FALSE. I feel like this is because my migration is not loaded (?) when I enable my module in the test, but I'm not sure what to do about it. What am I missing here? How can I load my migration in the test?

The migrate module provides a base Kernel test class, Drupal\Tests\migrate\Kernel\MigrateTestBase, which has a getMigration() method baked in.

For example, this works and the test passes:

<?php

namespace Drupal\my_module\tests\Kernel;

use Drupal\migrate\Plugin\MigrationInterface;
use Drupal\Tests\migrate\Kernel\MigrateTestBase;

class MyMigrationTest extends MigrateTestBase {

  protected static $modules = [
    'my_module',
    'migrate',
    // etc...
  ];
  
  public function testMigration(): void {
    $migration = $this->getMigration('MY_MIGRATION');

    $this->assertInstanceOf(MigrationInterface::class, $migration);
  }

}

However, it's not necessary to extend MigrateTestBase, this also works and test passes:

<?php

namespace Drupal\my_module\tests\Kernel;

use Drupal\KernelTests\KernelTestBase;
use Drupal\migrate\Plugin\MigrationInterface;

class MyMigrationTest extends KernelTestBase {

  protected static $modules = [
    'my_module',
    'migrate',
    // etc...
  ];
  
  public function testMigration(): void {
    /** @var \Drupal\migrate\Plugin\MigrationPluginManagerInterface 
    $plugin_manager */
    $plugin_manager = \Drupal::service('plugin.manager.migration');
    $migration = $plugin_manager
      ->createInstance('MY_MIGRATION');

    $this->assertInstanceOf(MigrationInterface::class, $migration);
  }

}