Added tests for SimpleSAML\Utils\Crypto

This also required adding an additional argument to
SimpleSAML\Utils\Crypto::loadPrivateKey to ease in testing. Without
this additional argument, SimpleSAML_Configuration::getBaseDir eventually
gets called to determine the private key location.  This doesn't work
well with vfsstream. This argument shouldn't cause too much trouble, and
seems cohesive enough with the function's purpose.