This reverts commit a4dcf65a.

* Support metadata XML directly in config
Also consider 'xml' as a key, in addition to 'url' and 'file'.
When provided, attempt to load this directly as a metadata XML string.
This is useful when working with dynamic configuration and external polling of metadata updates.

* Test loading XML from a fixed string
No preexisting test in this area
Adjust XML loader to only get global config in file mode - otherwise it is
unused, and it might pollute the test usage.
Test that an actully invalid config throws an exception, but that minimal valid
(albeit useless) XML is accepted.

Explicitly test a file that does not exist, as well as a preloaded one
Virtual test file names intented to be semantic but not collide.

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

A PHPUnit listener unsets SSP environmental variables and attempts
to restore globals.

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.