Show a message when the tests can't run at all (like WebGPU doesn't exist or requestAdapter fails)

[greggman]

This is in response to someone trying to run in compat mode and not knowing that they needed to use compatibility=1

I'm not sure where to put something like this. There's print_environment.spec.ts but I think that happens too late.

Effectively,

• if gpu doesn't exist we should print a message

  No WebGPU

• if requestAdapter returns null should print a message like

  Failed to get an adapter. If you meant to run in compatibility mode add compatibility=1
  to the URL or --compat in dawn.node or other tools

Since we wrap requestAdapter, maybe we could check this on first call to it? But I'm not sure how to surface the message.