Issue #2345833 by quietone, benjy: Convert assetEqual to assertIdentical in migrate_drupal

...migrate_drupal/src/Tests/d6/MigrateAggregatorFeedTest.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

...es/migrate_drupal/src/Tests/d6/MigrateCommentTypeTest.php

.../src/Tests/d6/MigrateCommentVariableEntityDisplayTest.php

...d6/MigrateCommentVariableEntityFormDisplaySubjectTest.php

.../Tests/d6/MigrateCommentVariableEntityFormDisplayTest.php

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

...igrate_drupal/src/Tests/d6/MigrateContactCategoryTest.php

...les/migrate_drupal/src/Tests/d6/MigrateDateFormatTest.php

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

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

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

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

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