15 files declare their use of PluginTestTrait in Search API 8

AjaxTest.php in tests/src/FunctionalJavascript/AjaxTest.php

ConfigOverrideIntegrationTest.php in tests/src/Functional/ConfigOverrideIntegrationTest.php

ConfigOverrideKernelTest.php in tests/src/Kernel/ConfigEntity/ConfigOverrideKernelTest.php

DefaultConfigEntityInstallationTest.php in tests/src/Kernel/ConfigEntity/DefaultConfigEntityInstallationTest.php

DependencyRemovalTest.php in tests/src/Kernel/ConfigEntity/DependencyRemovalTest.php

EventsTest.php in tests/src/Functional/EventsTest.php

HooksTest.php in tests/src/Functional/HooksTest.php

IndexChangesTest.php in tests/src/Kernel/Index/IndexChangesTest.php

IndexLoadItemsTest.php in tests/src/Kernel/Index/IndexLoadItemsTest.php

IntegrationTest.php in tests/src/Functional/IntegrationTest.php

ProcessorIntegrationTest.php in tests/src/Functional/ProcessorIntegrationTest.php

QueryTest.php in tests/src/Kernel/System/QueryTest.php

ServerChangesTest.php in tests/src/Kernel/Server/ServerChangesTest.php

ServerStorageTest.php in tests/src/Kernel/Server/ServerStorageTest.php

ServerTaskTest.php in tests/src/Kernel/Server/ServerTaskTest.php