validate: Add a mock decryptor element.
This is a mock element for testing the demuxing of CENC encoded MP4 files. Without such a mock element, qtdemux will fail since it can not find a suitable decryptor element in the registry.
While we can't have tests for the correct decryption of (say) Widevine encoded media files, we can have some useful testing on CENC boxes being correctly parsed by qtdemux.
This commit does not come with any use-cases, but a gst-validate test will be exercising it in order to add a test for gst-plugins-good!111 (merged)