Issue #2315489 by benjy: Renamed prepareIdMappings() in MigrateTestBase.

core/modules/migrate/src/Tests/MigrateTestBase.php

...migrate_drupal/src/Tests/d6/MigrateAggregatorItemTest.php

...s/migrate_drupal/src/Tests/d6/MigrateBlockContentTest.php

.../modules/migrate_drupal/src/Tests/d6/MigrateBlockTest.php

core/modules/migrate_drupal/src/Tests/d6/MigrateBookTest.php

...grate_drupal/src/Tests/d6/MigrateCckFieldRevisionTest.php

...migrate_drupal/src/Tests/d6/MigrateCckFieldValuesTest.php

...odules/migrate_drupal/src/Tests/d6/MigrateCommentTest.php

...drupal/src/Tests/d6/MigrateCommentVariableDisplayBase.php

...d6/MigrateCommentVariableEntityFormDisplaySubjectTest.php

...e_drupal/src/Tests/d6/MigrateCommentVariableFieldTest.php

...rupal/src/Tests/d6/MigrateCommentVariableInstanceTest.php

...migrate_drupal/src/Tests/d6/MigrateContactConfigsTest.php

...drupal/src/Tests/d6/MigrateFieldFormatterSettingsTest.php

.../migrate_drupal/src/Tests/d6/MigrateFieldInstanceTest.php

...te_drupal/src/Tests/d6/MigrateFieldWidgetSettingsTest.php

...s/migrate_drupal/src/Tests/d6/MigrateNodeRevisionTest.php

...dules/migrate_drupal/src/Tests/d6/MigrateNodeTestBase.php

.../migrate_drupal/src/Tests/d6/MigrateProfileValuesTest.php

...s/migrate_drupal/src/Tests/d6/MigrateTaxonomyTermTest.php