Issue #3096991 by quietone, alexpott, Wim Leers: Remove deprecated MigrateSqlSourceTestCase

...modules/migrate/src/Plugin/migrate/destination/Config.php

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

...al/src/Plugin/migrate/source/d6/D6VariableTranslation.php

...rate_drupal/src/Plugin/migrate/source/d6/i18nVariable.php

.../src/Kernel/Plugin/migrate/source/d6/i18nVariableTest.php

...drupal/tests/src/Unit/source/VariableMultiRowTestBase.php

...les/migrate_drupal/tests/src/Unit/source/VariableTest.php

...upal/tests/src/Unit/source/d6/VariableTranslationTest.php

...rate_drupal/tests/src/Unit/source/d6/i18nVariableTest.php