* Replace mock-test with actual test

Add support for RFC6265bis SameSite cookie attribute

This reverts commit 6bcea51e.

Stream wrappers were assumed to be relative paths. When using a stream wrapper (e.g. s3://) it was appended to the given base path.

The old mail class uses the built-in `mail` function in PHP, and builds a payload by itself using hard-coded HTML and MIME-snippets. Let's use a library for that; PHPMailer has new dependencies and does all the heavy lifting.

Also make the default namespace parameter mandatory, so that the function is not ADFS-specific.

Accessing a key in an array is not allowed when the array is not a variable but a value returned by a function.

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.