Fix local (non-http) metadata-locations

1. SimpleSAML\Test\Module\aggregator\Controller\AggregatorTest::testGetWithId

Symfony\Component\HttpClient\Exception\InvalidArgumentException: Invalid URL: scheme is missing in "tests/metadata/example.xml". Did you forget to add "http(s)://"?
/home/runner/work/simplesamlphp-module-aggregator2/simplesamlphp-module-aggregator2/vendor/symfony/http-client/HttpClientTrait.php:582
/home/runner/work/simplesamlphp-module-aggregator2/simplesamlphp-module-aggregator2/vendor/symfony/http-client/HttpClientTrait.php:179
/home/runner/work/simplesamlphp-module-aggregator2/simplesamlphp-module-aggregator2/vendor/symfony/http-client/CurlHttpClient.php:91
/home/runner/work/simplesamlphp-module-aggregator2/simplesamlphp-module-aggregator2/vendor/simplesamlphp/simplesamlphp/src/SimpleSAML/Utils/HTTP.php:554
/home/runner/work/simplesamlphp-module-aggregator2/simplesamlphp-module-aggregator2/src/EntitySource.php:150
/home/runner/work/simplesamlphp-module-aggregator2/simplesamlphp-module-aggregator2/src/EntitySource.php:228
/home/runner/work/simplesamlphp-module-aggregator2/simplesamlphp-module-aggregator2/src/EntitySource.php:263
/home/runner/work/simplesamlphp-module-aggregator2/simplesamlphp-module-aggregator2/src/Aggregator.php:514
/home/runner/work/simplesamlphp-module-aggregator2/simplesamlphp-module-aggregator2/src/Aggregator.php:693
/home/runner/work/simplesamlphp-module-aggregator2/simplesamlphp-module-aggregator2/src/Aggregator.php:727
/home/runner/work/simplesamlphp-module-aggregator2/simplesamlphp-module-aggregator2/src/Controller/Aggregator.php:98
/home/runner/work/simplesamlphp-module-aggregator2/simplesamlphp-module-aggregator2/tests/src/Controller/AggregatorTest.php:125

从 tests/src/Controller/AggregatorTest.php 开始并重现 testGetWithId，然后按照失败信息中的说明，通过 src/EntitySource.php 跟踪本地元数据路径。当 tests/metadata/example.xml 被接受为本地元数据位置，且 Symfony 不再报告缺少 URL 方案并且测试通过时，此更改就完成了。