MigrateTestBase::display() causes a TypeError if a test fails a migration

Problem/Motivation

In ::display() --

      $this->assertEquals('status', $type, $message);

PHPUnit now expects a string for $message.

But when MigrateExecutable does this:

      $this->message->display(
        $this->t('Migration failed with source plugin exception: @e in @file line @line', [
          '@e' => $e->getMessage(),
          '@file' => $e->getFile(),
          '@line' => $e->getLine(),
        ]), 'error');

It's calling MigrateTestBase::display() -- presumably because the test class is being used as a test version of whatever handles a message.

This means that if a migration fails in a test, the real failure is obscured by a TypeError.

Steps to reproduce

Proposed resolution

In \Drupal\Tests\migrate\Kernel\MigrateTestBase::display cast $message to a string.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet