tool for testing migration processing

It would be really useful to have a UI tool for testing migration process configuration on the fly.

At the moment, when something goes wrong with processing, you have to tweak the config file, do config-devel-import, run a migration again, watch it fail, repeat.

I am thinking a UI that has a form that lets you specify:
- the migration ID
- a single source ID to try to import
- a text area into which to paste process YML configuration

It's possible to pass process configuration to a migration on the fly, like this:

  $migration_id = 'mymigration';

  $manager = \Drupal::service('plugin.manager.config_entity_migration');
  $migration = $manager->createInstance($migration_id);

// This is anything that could go under 'process:' in a migration config file:
  $process_yaml = <<<EOT
type: field_foo/0/value
EOT;
  $process = [
    "type" => "field_section/0/value",
  ];
  $process = \Symfony\Component\Yaml\Yaml::parse($process_yaml);

  $migration->setProcess($process);

  $log = new Drupal\migrate_tools\DrushLogMigrateMessage();
  $executable = new Drupal\migrate_tools\MigrateExecutable($migration, $log, []);

  $source = $migration->getSourcePlugin();
  $source->rewind();
  $source->valid();

  $row = $source->current();
  dsm($row);
  $executable->processRow($row);
  dsm($row);

The second output of $row shows the processed data in its destination property.