test: Remove many expected files

For these tests, we only really care that we can successfully consume
them and generate valid output for each backend. Having the expected
files in the tree generates significant churn for any change to how we
generate backend code, which makes it hard to inspect diffs.

Change-Id: Ic98c248081144c0fb1791f1303eaf6d459548e3d
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/62720
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: James Price <jrprice@google.com>