Run tests against noop backend?

[kpreid]

Followup from review of #7063, which promotes the Empty backend (now renamed to Noop) to external usability as a test stub:

Should the tests be run against the noop adapter? This is trivial to do (just enable it in init::initialize_instance), but of course they almost all fail, because no rendering or compute is done. However, it might be useful to do so anyway, while skipping assertions about the output, in order to test the noop backend and wgpu's validation logic.

[cwfitzgerald]

I've thought about this a bit, I wonder if we can do this by having it opt in, like a enable_noop_tests to enable the test on the noop backend