Add tests.
This is going to be used by intrinsic-gen, a new tool
Bug: tint:832
Change-Id: Ib22ffa88a699be9c8dfdf3f8f6e32cff4ffc88e3
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/52602
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: David Neto <dneto@google.com>