This Cl splits apart the builder_impl test file in order to make it
easier to determine what has been tested.
Bug: tint:1718
Change-Id: Ib2f4d11a398b26fd774eb8c60555475053f20e0b
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/131581
Kokoro: Ben Clayton <bclayton@google.com>
Reviewed-by: James Price <jrprice@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>