Add more tests for SimpleSAML_Configuration::getDefaultEndpoint() and SimpleSAML\Utils\Config\Metadata::getDefaultEndpoint().

Make it possible to pre-load a configuration array into SimpleSAML_Configuration for subsequent calls to getInstance() to use it. This opens up for unit-testing code that calls SimpleSAML_Configuration::getInstance().

Add tests for most of the data retrieval functions. No tests
for the metadata functions present though.