Also spruce up texture validation tests so that they validate error
messages.
Bug: tint:805
Change-Id: I6c86fc16014b127a7ef8254e5badf9b5bed08623
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/51860
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>