Issue #2928710 by borisson_, dawehner, idebr: Eliminate usages of \Drupal::root() in tests

core/modules/big_pipe/tests/src/Functional/BigPipeTest.php

...les/field/tests/src/Kernel/FieldTypePluginManagerTest.php

...le/tests/src/Kernel/Migrate/d6/FileMigrationTestTrait.php

core/modules/image/tests/src/Kernel/ImageItemTest.php

...modules/image/tests/src/Kernel/ImageThemeFunctionTest.php

.../modules/media/tests/src/Functional/MediaRevisionTest.php

...media/tests/src/FunctionalJavascript/MediaDisplayTest.php

...a/tests/src/FunctionalJavascript/MediaSourceImageTest.php

.../modules/rdf/tests/src/Functional/StandardProfileTest.php

...odules/search/tests/src/Functional/SearchSimplifyTest.php

...ts/src/Functional/Common/EarlyRenderingControllerTest.php

...em/tests/src/Functional/FileTransfer/FileTransferTest.php

...ts/src/Functional/Update/DependencyHookInvocationTest.php

...tem/tests/src/Functional/Update/DependencyMissingTest.php

...em/tests/src/Functional/Update/DependencyOrderingTest.php

...tem/tests/src/Functional/Update/InvalidUpdateHookTest.php

...s/system/tests/src/Functional/Update/UpdateSchemaTest.php

.../system/tests/src/Functional/Update/UpdatesWith7xTest.php

...ules/system/tests/src/Kernel/Common/SystemListingTest.php

...ules/taxonomy/tests/src/Functional/VocabularyCrudTest.php