Adapt CTS to run on all available platforms.

[aarongreig]

The fixtures in the CTS are currently only capable of iterating over available devices within a single platform. If more than one platform is available on a system the user must select one of them to test via the --backend and --platform args, or by limiting which adapters are loaded with the UR_ADAPTERS_FORCE_LOAD env var. Refactoring the environment stuff and our fixtures to remove this limitation should be possible, and it would result in a nicer user experience (needing to get your args or environment in order before any tests will run is annoying), as well as a test suite which should be easier to integrate with the CI in https://github.com/intel/llvm.