Issue #2807917 by svendecabooter: Convert Node's Migrate source tests to new base class

...rc/Kernel/Plugin/migrate/source/d6/NodeByNodeTypeTest.php

...l/Plugin/migrate/source/d6/NodeRevisionByNodeTypeTest.php

.../src/Kernel/Plugin/migrate/source/d6/NodeRevisionTest.php

...de/tests/src/Kernel/Plugin/migrate/source/d6/NodeTest.php

...c/Kernel/Plugin/migrate/source/d6/NodeTranslationTest.php

...ests/src/Kernel/Plugin/migrate/source/d6/NodeTypeTest.php

...ests/src/Kernel/Plugin/migrate/source/d6/ViewModeTest.php

...de/tests/src/Kernel/Plugin/migrate/source/d7/NodeTest.php

...ests/src/Kernel/Plugin/migrate/source/d7/NodeTypeTest.php

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

...node/tests/src/Unit/Plugin/migrate/source/d6/NodeTest.php

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

.../tests/src/Unit/Plugin/migrate/source/d6/NodeTypeTest.php

.../tests/src/Unit/Plugin/migrate/source/d6/ViewModeTest.php

...node/tests/src/Unit/Plugin/migrate/source/d7/NodeTest.php

.../tests/src/Unit/Plugin/migrate/source/d7/NodeTypeTest.php