Use the new `cmdline.ts --coverage` flag in https://github.com/gpuweb/cts/pull/2206 to write out the per-test coverage to a unique file.
Change-Id: Iea273217eede2fa615b78cd6f69f036976dcfd35
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/117883
Kokoro: Ben Clayton <bclayton@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Ben Clayton <bclayton@chromium.org>