15 files declare their use of TestBase in Drupal 7 to 8/9 Module Upgrader 8

AnalyzerTestBase.php in tests/src/Unit/Plugin/DMU/Analyzer/AnalyzerTestBase.php

CreateClassTest.php in tests/src/Unit/Plugin/DMU/Fixer/CreateClassTest.php

DefineTest.php in tests/src/Unit/Plugin/DMU/Fixer/DefineTest.php

DeleteTest.php in tests/src/Unit/Plugin/DMU/Fixer/DeleteTest.php

DisableTest.php in tests/src/Unit/Plugin/DMU/Fixer/DisableTest.php

FormCallbackToMethodTest.php in tests/src/Unit/Plugin/DMU/Fixer/FormCallbackToMethodTest.php

FormStateTest.php in tests/src/Unit/Plugin/DMU/Rewriter/FormStateTest.php

FunctionCallModifierTestBase.php in tests/src/Unit/Plugin/DMU/Converter/Functions/FunctionCallModifierTestBase.php

GenericTest.php in tests/src/Unit/Plugin/DMU/Rewriter/GenericTest.php

HookToYAMLTest.php in tests/src/Unit/Plugin/DMU/Fixer/HookToYAMLTest.php

ImplementHookTest.php in tests/src/Unit/Plugin/DMU/Fixer/ImplementHookTest.php

ImplementTest.php in tests/src/Unit/Plugin/DMU/Fixer/ImplementTest.php

IndexerTestBase.php in tests/src/Unit/Plugin/DMU/Indexer/IndexerTestBase.php

NotifyTest.php in tests/src/Unit/Plugin/DMU/Fixer/NotifyTest.php

PSR4Test.php in tests/src/Unit/Plugin/DMU/Fixer/PSR4Test.php