Issue #2325575 by sun: Fixed Theme must not be primed in core.extension and KernelTestBase.

core/config/install/core.extension.yml

core/lib/Drupal/Core/Theme/DefaultNegotiator.php

core/lib/Drupal/Core/Theme/ThemeInitialization.php

core/lib/Drupal/Core/Theme/ThemeInitializationInterface.php

core/lib/Drupal/Core/Theme/ThemeManager.php

core/modules/simpletest/src/KernelTestBase.php

core/modules/simpletest/src/Tests/KernelTestBaseTest.php

core/modules/system/src/Tests/Extension/ThemeHandlerTest.php

core/tests/Drupal/Tests/Core/Extension/DefaultConfigTest.php