by tstoeckler: Add tests for local asset libraries

libraries.services.yml

src/ExternalLibrary/Asset/LocalRemoteAssetTrait.php

src/ExternalLibrary/Local/LocalLibraryTrait.php

src/Plugin/libraries/Locator/StreamLocator.php

src/StreamWrapper/AssetLibrariesStream.php

src/StreamWrapper/PhpFileLibrariesStream.php

tests/src/Kernel/ExternalLibrary/Asset/AssetLibraryTest.php

...src/Kernel/ExternalLibrary/PhpFile/PhpFileLibraryTest.php

...nel/ExternalLibrary/PhpFile/TestPhpLibraryFilesStream.php

tests/src/Kernel/ExternalLibrary/TestLibraryFilesStream.php

tests/src/Kernel/ExternalLibraryKernelTestBase.php