Issue #2573559 by phenaproxima, mikeryan: Remove MigrateDrupalTestBase::testSourcePlugin()

...ntent/tests/src/Unit/Plugin/migrate/source/d6/BoxTest.php

...sts/src/Unit/Plugin/migrate/source/d7/BlockCustomTest.php

...book/tests/src/Unit/Plugin/migrate/source/d6/BookTest.php

.../comment/src/Plugin/migrate/source/d6/CommentVariable.php

...src/Unit/Migrate/d6/CommentVariablePerCommentTypeTest.php

...comment/tests/src/Unit/Migrate/d6/CommentVariableTest.php

.../contact/src/Plugin/migrate/source/d6/ContactSettings.php

...src/Unit/Plugin/migrate/source/d6/ContactSettingsTest.php

...gin/migrate/source/d6/FieldInstancePerFormDisplayTest.php

...Plugin/migrate/source/d6/FieldInstancePerViewModeTest.php

...ules/file/src/Plugin/migrate/source/d6/UploadInstance.php

...file/tests/src/Unit/Plugin/migrate/source/d6/FileTest.php

.../src/Unit/Plugin/migrate/source/d6/UploadInstanceTest.php

...le/tests/src/Unit/Plugin/migrate/source/d6/UploadTest.php

...ules/image/src/Tests/Migrate/d6/MigrateImageCacheTest.php

...rc/Unit/Plugin/migrate/source/d6/ImageCachePresetTest.php

...dules/migrate/tests/src/Unit/MigrateSqlSourceTestCase.php

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

...odules/taxonomy/src/Plugin/migrate/source/d6/TermNode.php

...dules/taxonomy/tests/src/Unit/Migrate/d6/TermNodeTest.php