8 files declare their use of SearchPage in Drupal 9

ConfigEntityImportTest.php in core/modules/system/tests/src/Kernel/Entity/ConfigEntityImportTest.php

MigrateSearchPageTest.php in core/modules/search/tests/src/Kernel/Migrate/d6/MigrateSearchPageTest.php

MigrateSearchPageTest.php in core/modules/search/tests/src/Kernel/Migrate/d7/MigrateSearchPageTest.php

SearchConfigSettingsFormTest.php in core/modules/search/tests/src/Functional/SearchConfigSettingsFormTest.php

SearchPageRepositoryTest.php in core/modules/search/tests/src/Unit/SearchPageRepositoryTest.php

Contains \Drupal\Tests\search\Unit\SearchPageRepositoryTest.

SearchPageResourceTestBase.php in core/modules/search/tests/src/Functional/Rest/SearchPageResourceTestBase.php

SearchPageTest.php in core/modules/jsonapi/tests/src/Functional/SearchPageTest.php

SearchRankingTest.php in core/modules/search/tests/src/Functional/SearchRankingTest.php