Allow MigrateBatchExecutable to be subclassed

Without this, it's imposibble to override methods without duplicating batchProcessImport, because the methods won't be called.

This patch fixed the issue.

There is another one in batchImport, create and MR with additional change

Looks good to me

The failing test is present in HEAD - https://www.drupal.org/pift-ci-job/2820117 →

Thanks for this improvement

Automatically closed - issue fixed for 2 weeks with no activity.