gltestsrc: Implement URIHandler interface

This allows the element to be used in playbin, for instance.
3 jobs for philn/gltestsrc in 7 minutes and 19 seconds (queued for 2 minutes and 28 seconds)
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  Preparation
passed #53485
manifest

00:00:21

 
  Build
passed #53486
build fedora x86_64

00:02:33

 
  Full Builds
passed #53487
build android arm64 api28

00:04:25