Update src/tint unittests to new @stage format.

This CL updates all of the Tint unittests to the new @stage shorter
syntax. This also updates the WGSL writer to emit the new short forms
instead of using the deprecated form.

Bug: tint:1503
Change-Id: I8c49e5319a19cccb5b4b5078f3ab39c50f31a9a8
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/92483
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>