Issue #2514168 by phenaproxima, mikeryan, benjy: Streamline migrate_drupal integration tests

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

...odules/migrate_drupal/src/Tests/MigrateDrupalTestBase.php

.../migrate_drupal/src/Tests/d6/MigrateActionConfigsTest.php

...rate_drupal/src/Tests/d6/MigrateAggregatorConfigsTest.php

...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

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

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

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

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

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

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

...d6/MigrateCommentVariableEntityFormDisplaySubjectTest.php

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

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

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

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

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