Issue #2413759 by benjy, hosef: Move D6 dumps to avoid collisions with D7 dumps

.../modules/migrate_drupal/src/Tests/Dump/DrupalDumpBase.php

core/modules/migrate_drupal/src/Tests/Table/d6/Access.php

core/modules/migrate_drupal/src/Tests/Table/d6/Actions.php

.../modules/migrate_drupal/src/Tests/Table/d6/ActionsAid.php

.../migrate_drupal/src/Tests/Table/d6/AggregatorCategory.php

...ules/migrate_drupal/src/Tests/Table/d6/AggregatorFeed.php

...ules/migrate_drupal/src/Tests/Table/d6/AggregatorItem.php

core/modules/migrate_drupal/src/Tests/Table/d6/Authmap.php

core/modules/migrate_drupal/src/Tests/Table/d6/Batch.php

core/modules/migrate_drupal/src/Tests/Table/d6/Blocks.php

...modules/migrate_drupal/src/Tests/Table/d6/BlocksRoles.php

core/modules/migrate_drupal/src/Tests/Table/d6/Book.php

core/modules/migrate_drupal/src/Tests/Table/d6/Boxes.php

core/modules/migrate_drupal/src/Tests/Table/d6/Comments.php

core/modules/migrate_drupal/src/Tests/Table/d6/Config.php

core/modules/migrate_drupal/src/Tests/Table/d6/Contact.php

...s/migrate_drupal/src/Tests/Table/d6/ContentFieldImage.php

...rate_drupal/src/Tests/Table/d6/ContentFieldMultivalue.php

...es/migrate_drupal/src/Tests/Table/d6/ContentFieldTest.php

...migrate_drupal/src/Tests/Table/d6/ContentFieldTestTwo.php